feat(ics): export — share single event + back up local calendars as .ics

Branch 1 of 2 for v2.7 (the .ics topic). Adds the write side of a hand-rolled RFC 5545 engine (zero deps, stays on kotlinx-datetime): - domain/ics: IcsText (escape + 75-octet folding), IcsEvent model, IcsWriter.writeCalendar. Timezone rule: all-day VALUE=DATE, one-off timed UTC Z, recurring timed TZID-labelled from EVENT_TIMEZONE (no VTIMEZONE — import resolves TZID against the OS tz db). - Single-event share from the detail screen (FileProvider + ACTION_SEND). - Whole-calendar backup of the writable local calendars to a SAF file (Settings -> Calendars -> Export as .ics), one combined VCALENDAR. - insertEvent now writes Events.UID_2445; legacy rows fall back to a stable synthesised UID at export time so a later restore won't dupe. - EXDATE / RECURRENCE-ID overrides are deliberately skipped this pass (documented v1 limit; import will skip them too). Engine + mapper unit-tested. Import (Branch 2, feat/ics-import) ships in the same v2.7 release; no tag until both land + on-device review. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
2026-06-18 14:27:53 +02:00
parent 64d0a89b28
commit 0b683d374f
25 changed files with 1190 additions and 13 deletions
--- a/.planning/ROADMAP.md
+++ b/.planning/ROADMAP.md
@@ -233,18 +233,25 @@ pass on the existing controls; new toggles ride in with their own features.
 8. App shortcuts: ~~launcher long-press → New event~~ *(done, v2.5.0)*; optional quick-settings tile still open

 **Tier 4 — reliability, data-safety & interop** *(re-ranked 2026-06-17)*
-9. **Reminders — defaults + delivery reliability** *(next)* — global default
-   reminder **+ per-calendar override**, bundled with exact-alarm / battery
-   hardening. Elevated above .ics: it's core to the "Calendula is your only
-   calendar app" promise. Full sketch in "Reminders — defaults & delivery
-   reliability" below.
-10. **Local-calendar backup / export** — device-only calendars have no sync and
-    therefore **no backup**; losing the phone = total data loss. Whole-calendar
-    `.ics` export + restore. A data-integrity gap, not a feature; front-runs and
-    overlaps the single-event .ics work below.
-11. Share event as .ics + receive/open .ics into a prefilled create form
+9. **Reminders — defaults + delivery reliability** *(shipped v2.6.0)* — global
+   default reminder **+ per-calendar override**, bundled with battery-exemption
+   hardening. Full sketch in "Reminders — defaults & delivery reliability" below.
+10. **The `.ics` engine — export + import** *(in progress → v2.7)* — one
+    hand-rolled serializer/parser (zero deps, stays on `kotlinx-datetime`),
+    four surfaces: single-event share + whole-calendar backup (export),
+    open-`.ics`→form + whole-calendar restore (import). Closes the
+    device-local-calendar data-loss gap (#10/#11 merged here). Built as **two
+    sequential branches in one release**: `feat/ics-export` (write side +
+    UID-on-create precursor) then `feat/ics-import` (parser, restore, dedup).
+    Import is liberal-in/strict-out: skip-and-report foreign `VTIMEZONE` /
+    `RECURRENCE-ID` it can't model. Timezone rule: all-day `VALUE=DATE`,
+    non-recurring timed UTC `Z`, recurring timed `TZID`-labelled from the stored
+    `EVENT_TIMEZONE` (no `VTIMEZONE` blocks; resolved against the OS tz DB on
+    import). Plan: `docs/superpowers/plans/2026-06-18-05-ics-export.md`.
+11. **Snooze / dismiss notification actions** *(next, after v2.7)* — follows the
+    `.ics` work; inherits v2.6's deferred exact-alarm/WorkManager decision (snooze
+    must re-fire an alarm).
 12. Drag & drop rescheduling in day/week — big-ticket, own slice (recurring drops reuse the scope dialog)
-13. Snooze / dismiss notification actions — follows the reminders slice (#9)

 **Gated — explicit go/no-go before any work (mostly INTERNET-permission calls)**
 - Remote calendar create/edit (re-implements DAVx5; INTERNET + credential storage)
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,17 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0

 ## [Unreleased]

+### Added
+- Share a single event as an `.ics` file from the event detail screen — hands a
+  standard calendar file to any app via the system share sheet.
+- Back up your local (device-only) calendars: Settings → Calendars → Export as
+  `.ics` file writes every event of your on-device calendars to a file you
+  choose. Local calendars aren't synced anywhere, so this is their only backup.
+  (Importing `.ics` files back in lands in the next update.)
+
+_Note: new events now carry a unique identifier so a future `.ics` import can
+recognise them and avoid duplicates._
+
 ## [2.6.0] — 2026-06-18

 ### Added
--- a/app/src/main/AndroidManifest.xml
+++ b/app/src/main/AndroidManifest.xml
@@ -112,6 +112,19 @@
            </intent-filter>
        </receiver>

+        <!-- Hands .ics files we stage in the cache to other apps via a content
+             Uri (single-event share). Authority tracks applicationId so the
+             debug suffix doesn't break getUriForFile. -->
+        <provider
+            android:name="androidx.core.content.FileProvider"
+            android:authorities="${applicationId}.fileprovider"
+            android:exported="false"
+            android:grantUriPermissions="true">
+            <meta-data
+                android:name="android.support.FILE_PROVIDER_PATHS"
+                android:resource="@xml/file_paths" />
+        </provider>
+
        <!-- Persists the per-app language (M4) on API < 33, where the platform
             per-app-languages API is unavailable. On 33+ this is a no-op. -->
        <service
--- a/app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/CalendarDataSource.kt
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/CalendarDataSource.kt
@@ -19,10 +19,12 @@ import de.jeanlucmakiola.calendula.domain.EventDetail
 import de.jeanlucmakiola.calendula.domain.EventForm
 import de.jeanlucmakiola.calendula.domain.EventInstance
 import de.jeanlucmakiola.calendula.domain.Reminder
+import de.jeanlucmakiola.calendula.domain.ics.IcsEvent
 import de.jeanlucmakiola.calendula.domain.rruleTruncatedAt
 import kotlinx.datetime.toJavaLocalDate
 import java.time.ZoneId
 import java.time.ZoneOffset
+import java.util.UUID
 import javax.inject.Inject
 import javax.inject.Singleton

@@ -48,6 +50,13 @@ interface CalendarDataSource {
     */
    fun eventColorPalette(calendarId: Long): List<EventColorOption>

+    /**
+     * Every master/one-off event of the writable local calendars, mapped for a
+     * whole-calendar `.ics` backup. Modified-occurrence and cancelled-exception
+     * rows are excluded (see [EventExportProjection]).
+     */
+    fun exportableEvents(): List<IcsEvent>
+
    /**
     * Create a new device-only (`ACCOUNT_TYPE_LOCAL`) calendar the app owns;
     * returns its `Calendars._ID`. Inserted through the sync-adapter URI so the
@@ -265,6 +274,39 @@ class AndroidCalendarDataSource @Inject constructor(
            ?: emptyList()
    }

+    override fun exportableEvents(): List<IcsEvent> {
+        // Only the local calendars the app owns and can write — synced calendars
+        // already have a backup (their server). Map id → display name for the
+        // X-CALENDULA-CALENDAR tag a restore uses to fan back out.
+        val names = calendars()
+            .filter { it.isLocal && it.canModifyContents }
+            .associate { it.id to it.displayName }
+        if (names.isEmpty()) return emptyList()
+
+        val idList = names.keys.joinToString(",")
+        return resolver.query(
+            CalendarContract.Events.CONTENT_URI,
+            EventExportProjection.COLUMNS,
+            // Skip soft-deleted rows and exception rows (modified occurrences /
+            // cancellations) — v1 exports masters + one-offs only.
+            "${CalendarContract.Events.CALENDAR_ID} IN ($idList) AND " +
+                "${CalendarContract.Events.DELETED} = 0 AND " +
+                "${CalendarContract.Events.ORIGINAL_ID} IS NULL",
+            null,
+            CalendarContract.Events.DTSTART + " ASC",
+        )?.use { c ->
+            c.mapAll {
+                val reader = CursorColumnReader(c)
+                val eventId = reader.getLong(EventExportProjection.IDX_ID)
+                val calendarId = reader.getLong(EventExportProjection.IDX_CALENDAR_ID)
+                reader.toIcsEvent(
+                    reminderMinutes = queryReminders(eventId).map { it.minutes },
+                    calendarName = names[calendarId],
+                )
+            }
+        } ?: emptyList()
+    }
+
    /** The account a calendar belongs to, for scoping a `Colors` lookup. */
    private fun calendarAccount(calendarId: Long): CalendarAccount? = resolver.query(
        ContentUris.withAppendedId(CalendarContract.Calendars.CONTENT_URI, calendarId),
@@ -316,6 +358,11 @@ class AndroidCalendarDataSource @Inject constructor(
                CalendarContract.Events.CALENDAR_ID,
                requireNotNull(form.calendarId) { "EventForm.calendarId is required" },
            )
+            // A globally-unique UID so a later .ics backup/restore can identify
+            // the event and not duplicate it on re-import (the provider leaves
+            // this null for events it didn't sync). Older rows without one fall
+            // back to a stable synthesised UID at export time (deriveIcsUid).
+            put(CalendarContract.Events.UID_2445, "${UUID.randomUUID()}@calendula")
            put(CalendarContract.Events.TITLE, form.title.trim())
            put(CalendarContract.Events.ALL_DAY, if (form.isAllDay) 1 else 0)
            put(CalendarContract.Events.DTSTART, times.dtStartMillis)
--- a/app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/CalendarRepository.kt
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/CalendarRepository.kt
@@ -5,6 +5,7 @@ import de.jeanlucmakiola.calendula.domain.EventColorOption
 import de.jeanlucmakiola.calendula.domain.EventDetail
 import de.jeanlucmakiola.calendula.domain.EventForm
 import de.jeanlucmakiola.calendula.domain.EventInstance
+import de.jeanlucmakiola.calendula.domain.ics.IcsEvent
 import kotlinx.coroutines.flow.Flow
 import kotlin.time.Instant

@@ -28,6 +29,12 @@ interface CalendarRepository {
    /** Permanently delete a local calendar the app owns, with all its events. */
    suspend fun deleteCalendar(id: Long)

+    /**
+     * Every event of the writable local calendars, ready to serialise into a
+     * whole-calendar `.ics` backup (see [CalendarDataSource.exportableEvents]).
+     */
+    suspend fun exportEvents(): List<IcsEvent>
+
    /** Create a new event from a validated form; returns the new `Events._ID`. */
    suspend fun createEvent(form: EventForm): Long

--- a/app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/CalendarRepositoryImpl.kt
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/CalendarRepositoryImpl.kt
@@ -99,6 +99,8 @@ class CalendarRepositoryImpl @Inject constructor(
    override suspend fun deleteCalendar(id: Long) =
        withContext(io) { dataSource.deleteCalendar(id) }

+    override suspend fun exportEvents() = withContext(io) { dataSource.exportableEvents() }
+
    override suspend fun createEvent(form: EventForm): Long = withContext(io) {
        dataSource.insertEvent(form, allDayReminderTimeMinutes())
    }
--- a/app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/IcsExportMapper.kt
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/IcsExportMapper.kt
@@ -0,0 +1,91 @@
+package de.jeanlucmakiola.calendula.data.calendar
+
+import de.jeanlucmakiola.calendula.domain.EventStatus
+import de.jeanlucmakiola.calendula.domain.ics.IcsEvent
+import de.jeanlucmakiola.calendula.domain.ics.deriveIcsUid
+
+/**
+ * Map one Events row (read through [EventExportProjection]) into an [IcsEvent]
+ * for backup. [reminderMinutes] are the row's raw provider reminder offsets and
+ * [calendarName] the display name of its calendar (emitted as
+ * `X-CALENDULA-CALENDAR`). Pure given a [ColumnReader] — JVM-tested with
+ * MapColumnReader.
+ */
+internal fun ColumnReader.toIcsEvent(
+    reminderMinutes: List<Int>,
+    calendarName: String?,
+): IcsEvent {
+    val eventId = getLong(EventExportProjection.IDX_ID)
+    val dtStart = getLong(EventExportProjection.IDX_DTSTART)
+    val rrule = getString(EventExportProjection.IDX_RRULE)?.takeIf { it.isNotBlank() }
+
+    // Recurring rows store DURATION instead of DTEND; reconstruct the end from it
+    // so the writer can render DTEND. A missing/blank both means a zero-length event.
+    val end = when {
+        !isNull(EventExportProjection.IDX_DTEND) -> getLong(EventExportProjection.IDX_DTEND)
+        else -> dtStart + parseRfc2445DurationMillis(getString(EventExportProjection.IDX_DURATION))
+    }
+
+    // STATUS shares value 0 with TENTATIVE, so an absent column must read as Confirmed.
+    val status = if (isNull(EventExportProjection.IDX_STATUS)) {
+        EventStatus.Confirmed
+    } else {
+        mapEventStatus(getInt(EventExportProjection.IDX_STATUS))
+    }
+
+    return IcsEvent(
+        uid = deriveIcsUid(getString(EventExportProjection.IDX_UID), eventId, dtStart),
+        summary = getString(EventExportProjection.IDX_TITLE).orEmpty(),
+        start = dtStart.toKotlinInstantFromEpochMillis(),
+        end = end.toKotlinInstantFromEpochMillis(),
+        isAllDay = getInt(EventExportProjection.IDX_ALL_DAY) != 0,
+        zoneId = getString(EventExportProjection.IDX_EVENT_TIMEZONE)?.takeIf { it.isNotBlank() }
+            ?: "UTC",
+        recurrenceRule = rrule,
+        location = getString(EventExportProjection.IDX_LOCATION),
+        description = getString(EventExportProjection.IDX_DESCRIPTION),
+        reminderMinutes = reminderMinutes,
+        status = status,
+        availability = mapAvailability(getInt(EventExportProjection.IDX_AVAILABILITY)),
+        calendarName = calendarName,
+    )
+}
+
+// Android's calendar provider (and Calendula's own writes) use the non-standard
+// single-unit forms P<n>S / P<n>D / P<n>W — seconds without the RFC-required
+// leading T. Matched first; anything else falls through to the general grammar.
+private val DURATION_SINGLE_UNIT = Regex("""([+-]?)P(\d+)([WDS])""")
+private val DURATION_GENERAL =
+    Regex("""([+-]?)P(?:(\d+)W)?(?:(\d+)D)?(?:T(?:(\d+)H)?(?:(\d+)M)?(?:(\d+)S)?)?""")
+
+/**
+ * Milliseconds of a DURATION (`P1D`, `P3600S`, `PT1H30M`, `P1W`, …). Calendula
+ * only ever writes `P<days>D` / `P<seconds>S`, but the parser also handles the
+ * general RFC 5545 grammar so a foreign row on a local calendar still
+ * round-trips. Unparseable input is treated as zero.
+ */
+internal fun parseRfc2445DurationMillis(duration: String?): Long {
+    if (duration.isNullOrBlank()) return 0L
+    val s = duration.trim()
+
+    DURATION_SINGLE_UNIT.matchEntire(s)?.let { m ->
+        val unitSeconds = when (m.groupValues[3]) {
+            "W" -> 7L * 24 * 60 * 60
+            "D" -> 24L * 60 * 60
+            else -> 1L // S
+        }
+        return m.signum() * m.groupValues[2].toLong() * unitSeconds * 1_000L
+    }
+
+    val m = DURATION_GENERAL.matchEntire(s) ?: return 0L
+    val weeks = m.groupValues[2].toLongOrNull() ?: 0L
+    val days = m.groupValues[3].toLongOrNull() ?: 0L
+    val hours = m.groupValues[4].toLongOrNull() ?: 0L
+    val minutes = m.groupValues[5].toLongOrNull() ?: 0L
+    val seconds = m.groupValues[6].toLongOrNull() ?: 0L
+    val totalSeconds = ((((weeks * 7 + days) * 24 + hours) * 60 + minutes) * 60 + seconds)
+    return m.signum() * totalSeconds * 1_000L
+}
+
+/** Sign carried by group 1 (`-` → -1, otherwise +1) of a duration match. */
+private fun MatchResult.signum(): Long = if (groupValues[1] == "-") -1L else 1L
--- a/app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/Projections.kt
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/Projections.kt
@@ -97,6 +97,48 @@ internal object EventDetailProjection {
    const val IDX_EVENT_COLOR_KEY = 17
 }

+/**
+ * Master/one-off Events rows for a whole-calendar backup. Unlike
+ * [EventDetailProjection] this reads `UID_2445` (to keep a row's identity across
+ * backups) and `DURATION` (recurring rows carry it instead of DTEND). Modified-
+ * occurrence and cancelled-exception rows are filtered out by the query
+ * (`ORIGINAL_ID IS NULL`), so RECURRENCE-ID overrides and EXDATEs aren't
+ * exported yet — a documented v1 limit (import skips them too).
+ */
+internal object EventExportProjection {
+    val COLUMNS: Array<String> = arrayOf(
+        CalendarContract.Events._ID,
+        CalendarContract.Events.UID_2445,
+        CalendarContract.Events.TITLE,
+        CalendarContract.Events.DTSTART,
+        CalendarContract.Events.DTEND,
+        CalendarContract.Events.DURATION,
+        CalendarContract.Events.ALL_DAY,
+        CalendarContract.Events.EVENT_TIMEZONE,
+        CalendarContract.Events.RRULE,
+        CalendarContract.Events.EVENT_LOCATION,
+        CalendarContract.Events.DESCRIPTION,
+        CalendarContract.Events.STATUS,
+        CalendarContract.Events.AVAILABILITY,
+        CalendarContract.Events.CALENDAR_ID,
+    )
+
+    const val IDX_ID = 0
+    const val IDX_UID = 1
+    const val IDX_TITLE = 2
+    const val IDX_DTSTART = 3
+    const val IDX_DTEND = 4
+    const val IDX_DURATION = 5
+    const val IDX_ALL_DAY = 6
+    const val IDX_EVENT_TIMEZONE = 7
+    const val IDX_RRULE = 8
+    const val IDX_LOCATION = 9
+    const val IDX_DESCRIPTION = 10
+    const val IDX_STATUS = 11
+    const val IDX_AVAILABILITY = 12
+    const val IDX_CALENDAR_ID = 13
+}
+
 internal object AttendeeProjection {
    val COLUMNS: Array<String> = arrayOf(
        CalendarContract.Attendees.ATTENDEE_NAME,
--- a/app/src/main/java/de/jeanlucmakiola/calendula/data/ics/IcsExporter.kt
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/data/ics/IcsExporter.kt
@@ -0,0 +1,45 @@
+package de.jeanlucmakiola.calendula.data.ics
+
+import android.content.Context
+import android.net.Uri
+import androidx.core.content.FileProvider
+import dagger.hilt.android.qualifiers.ApplicationContext
+import java.io.File
+import java.io.IOException
+import javax.inject.Inject
+import javax.inject.Singleton
+
+/**
+ * The Android IO edge of `.ics` export: writes a serialised calendar to a
+ * SAF document (whole-calendar backup) or stages it in a cache file behind a
+ * `FileProvider` content Uri (single-event share). The serialisation itself is
+ * the pure `domain.ics.IcsWriter`; this class only moves bytes.
+ */
+@Singleton
+class IcsExporter @Inject constructor(
+    @ApplicationContext private val context: Context,
+) {
+
+    /** Write [content] to the SAF document at [uri] as UTF-8. Throws on failure. */
+    fun writeDocument(uri: Uri, content: String) {
+        context.contentResolver.openOutputStream(uri)?.use { out ->
+            out.write(content.toByteArray(Charsets.UTF_8))
+        } ?: throw IOException("Could not open $uri for writing")
+    }
+
+    /**
+     * Stage [content] in a private cache file and return a shareable content
+     * Uri for an `ACTION_SEND`. [fileName] is the suggested `.ics` name shown to
+     * the receiving app. The authority mirrors the manifest's `FileProvider`.
+     */
+    fun stageShareFile(fileName: String, content: String): Uri {
+        val dir = File(context.cacheDir, SHARE_DIR).apply { mkdirs() }
+        val file = File(dir, fileName)
+        file.writeText(content, Charsets.UTF_8)
+        return FileProvider.getUriForFile(context, "${context.packageName}.fileprovider", file)
+    }
+
+    private companion object {
+        const val SHARE_DIR = "shared_ics"
+    }
+}
--- a/app/src/main/java/de/jeanlucmakiola/calendula/domain/ics/EventDetailIcs.kt
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/domain/ics/EventDetailIcs.kt
@@ -0,0 +1,29 @@
+package de.jeanlucmakiola.calendula.domain.ics
+
+import de.jeanlucmakiola.calendula.domain.EventDetail
+
+/**
+ * Build the [IcsEvent] for sharing a single event. We export the event the user
+ * is looking at as a **one-off** VEVENT (no RRULE): the detail screen shows one
+ * occurrence, so "share this event" should hand off exactly that instance, not
+ * a whole series anchored to a possibly-different DTSTART. Reminders are the
+ * already-decoded semantic lead times the detail screen holds.
+ */
+fun EventDetail.toShareIcsEvent(): IcsEvent {
+    val startMillis = instance.start.toEpochMilliseconds()
+    return IcsEvent(
+        uid = deriveIcsUid(existingUid = null, eventId = instance.eventId, dtStartMillis = startMillis),
+        summary = instance.title,
+        start = instance.start,
+        end = instance.end,
+        isAllDay = instance.isAllDay,
+        zoneId = eventTimezone?.takeIf { it.isNotBlank() } ?: "UTC",
+        recurrenceRule = null,
+        location = instance.location,
+        description = description,
+        reminderMinutes = reminders.map { it.minutes },
+        status = status,
+        availability = availability,
+        calendarName = null,
+    )
+}
--- a/app/src/main/java/de/jeanlucmakiola/calendula/domain/ics/IcsEvent.kt
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/domain/ics/IcsEvent.kt
@@ -0,0 +1,43 @@
+package de.jeanlucmakiola.calendula.domain.ics
+
+import de.jeanlucmakiola.calendula.domain.Availability
+import de.jeanlucmakiola.calendula.domain.EventStatus
+import kotlin.time.Instant
+
+/**
+ * A single event ready to be serialised to a `VEVENT`, decoupled from the
+ * provider so [IcsWriter] stays pure-Kotlin and JVM-testable. [start]/[end] are
+ * absolute instants; [isAllDay] and [recurrenceRule] decide how they are
+ * rendered (see [IcsWriter]'s timezone rule).
+ */
+data class IcsEvent(
+    /** RFC 5545 UID — globally stable across exports (see [deriveIcsUid]). */
+    val uid: String,
+    val summary: String,
+    val start: Instant,
+    /** Exclusive end (for all-day: the next-day midnight, as the provider stores it). */
+    val end: Instant,
+    val isAllDay: Boolean,
+    /** IANA zone id (`Events.EVENT_TIMEZONE`); only used for recurring timed events. */
+    val zoneId: String,
+    /** Bare RRULE value (no `RRULE:` prefix), or null for a one-off event. */
+    val recurrenceRule: String? = null,
+    val location: String? = null,
+    val description: String? = null,
+    /** Reminder lead times in minutes before start (raw provider offsets). */
+    val reminderMinutes: List<Int> = emptyList(),
+    val status: EventStatus = EventStatus.Confirmed,
+    val availability: Availability = Availability.Busy,
+    /** Source calendar name, emitted as `X-CALENDULA-CALENDAR` so a combined backup can fan back out. */
+    val calendarName: String? = null,
+)
+
+/**
+ * The UID to export for a provider event. A row that already carries a UID
+ * (`Events.UID_2445`) keeps it; otherwise we synthesise a **stable** one from
+ * the event id and its DTSTART so the same legacy event yields the same UID
+ * across repeated backups — which keeps a later restore from duplicating it.
+ */
+fun deriveIcsUid(existingUid: String?, eventId: Long, dtStartMillis: Long): String =
+    existingUid?.trim()?.takeIf { it.isNotEmpty() }
+        ?: "$eventId-$dtStartMillis@calendula"
--- a/app/src/main/java/de/jeanlucmakiola/calendula/domain/ics/IcsText.kt
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/domain/ics/IcsText.kt
@@ -0,0 +1,62 @@
+package de.jeanlucmakiola.calendula.domain.ics
+
+/**
+ * Low-level RFC 5545 text mechanics, kept separate from [IcsWriter] so the
+ * escaping and folding rules can be tested in isolation. Pure Kotlin — no
+ * Android, no time handling.
+ */
+
+/** iCalendar mandates CRLF line breaks, not the platform separator. */
+const val ICS_CRLF: String = "\r\n"
+
+/** RFC 5545 §3.1: content lines SHOULD be folded at 75 octets (excluding the break). */
+private const val MAX_OCTETS = 75
+
+/**
+ * Escape a TEXT value (SUMMARY, DESCRIPTION, LOCATION, …) per RFC 5545 §3.3.11:
+ * backslash, semicolon and comma are escaped, newlines become the literal `\n`.
+ * Backslash is handled first so it doesn't double-escape the others' markers.
+ */
+fun escapeText(value: String): String = buildString(value.length) {
+    for (ch in value) {
+        when (ch) {
+            '\\' -> append("\\\\")
+            ';' -> append("\\;")
+            ',' -> append("\\,")
+            '\n' -> append("\\n")
+            '\r' -> Unit // CR is dropped; a lone CRLF in source text folds to one \n
+            else -> append(ch)
+        }
+    }
+}
+
+/**
+ * Fold a single content line to ≤75 octets per physical line, inserting
+ * `CRLF + space` between segments (the space is part of the 75-octet budget of
+ * the continuation line, so its content caps at 74). Folding counts UTF-8
+ * octets, never splitting a multi-byte character across a boundary.
+ */
+fun foldLine(line: String): String {
+    if (line.toByteArray(Charsets.UTF_8).size <= MAX_OCTETS) return line
+    val out = StringBuilder()
+    var octetsThisLine = 0
+    var first = true
+    var i = 0
+    while (i < line.length) {
+        val cp = line.codePointAt(i)
+        val width = Character.charCount(cp)
+        val piece = line.substring(i, i + width)
+        val pieceOctets = piece.toByteArray(Charsets.UTF_8).size
+        // Continuation lines spend one octet on the leading space.
+        val budget = if (first) MAX_OCTETS else MAX_OCTETS - 1
+        if (octetsThisLine + pieceOctets > budget) {
+            out.append(ICS_CRLF).append(' ')
+            octetsThisLine = 0
+            first = false
+        }
+        out.append(piece)
+        octetsThisLine += pieceOctets
+        i += width
+    }
+    return out.toString()
+}
--- a/app/src/main/java/de/jeanlucmakiola/calendula/domain/ics/IcsWriter.kt
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/domain/ics/IcsWriter.kt
@@ -0,0 +1,124 @@
+package de.jeanlucmakiola.calendula.domain.ics
+
+import de.jeanlucmakiola.calendula.domain.Availability
+import de.jeanlucmakiola.calendula.domain.EventStatus
+import kotlinx.datetime.LocalDateTime
+import kotlinx.datetime.TimeZone
+import kotlinx.datetime.number
+import kotlinx.datetime.toLocalDateTime
+import kotlin.time.Instant
+
+/** Default `PRODID` advertising the writer that produced the file. */
+const val ICS_PROD_ID: String = "-//Jean-Luc Makiola//Calendula//EN"
+
+/**
+ * Hand-rolled RFC 5545 serialiser for the subset Calendula models. No iCal
+ * library: we stay on `kotlinx-datetime` and own the output, exactly as
+ * `domain/Recurrence.kt` owns RRULE. Pure and JVM-testable — the caller
+ * supplies [dtStamp] (the export moment) so the writer never reads a clock.
+ *
+ * Timezone rule (see plan 05, decision 1):
+ *  - all-day      → `VALUE=DATE`, no zone;
+ *  - timed one-off → UTC instant with a `Z` suffix (an instant is an instant);
+ *  - timed recurring → `TZID`-labelled local wall time, so the series stays
+ *    anchored to wall-clock across DST. No `VTIMEZONE` block is emitted; import
+ *    resolves the `TZID` against the OS tz database.
+ */
+class IcsWriter(private val prodId: String = ICS_PROD_ID) {
+
+    fun writeCalendar(events: List<IcsEvent>, dtStamp: Instant): String {
+        val lines = buildList {
+            add("BEGIN:VCALENDAR")
+            add("VERSION:2.0")
+            add("PRODID:$prodId")
+            add("CALSCALE:GREGORIAN")
+            events.forEach { appendEvent(it, dtStamp) }
+            add("END:VCALENDAR")
+        }
+        return lines.joinToString(ICS_CRLF, postfix = ICS_CRLF) { foldLine(it) }
+    }
+
+    private fun MutableList<String>.appendEvent(event: IcsEvent, dtStamp: Instant) {
+        add("BEGIN:VEVENT")
+        add("UID:${event.uid}")
+        add("DTSTAMP:${utcStamp(dtStamp)}")
+        add("SUMMARY:${escapeText(event.summary)}")
+        appendTimes(event)
+        event.recurrenceRule?.takeIf { it.isNotBlank() }
+            ?.let { add("RRULE:${it.removePrefix("RRULE:")}") }
+        event.location?.takeIf { it.isNotBlank() }
+            ?.let { add("LOCATION:${escapeText(it)}") }
+        event.description?.takeIf { it.isNotBlank() }
+            ?.let { add("DESCRIPTION:${escapeText(it)}") }
+        add("STATUS:${statusValue(event.status)}")
+        add("TRANSP:${transpValue(event.availability)}")
+        event.calendarName?.takeIf { it.isNotBlank() }
+            ?.let { add("X-CALENDULA-CALENDAR:${escapeText(it)}") }
+        event.reminderMinutes.filter { it >= 0 }.distinct().forEach { minutes ->
+            appendAlarm(minutes, event.summary)
+        }
+        add("END:VEVENT")
+    }
+
+    private fun MutableList<String>.appendTimes(event: IcsEvent) = when {
+        event.isAllDay -> {
+            add("DTSTART;VALUE=DATE:${utcDate(event.start)}")
+            add("DTEND;VALUE=DATE:${utcDate(event.end)}")
+        }
+        // Recurring: anchor to wall-clock in the event's own zone.
+        event.recurrenceRule?.isNotBlank() == true -> {
+            val zone = runCatching { TimeZone.of(event.zoneId) }.getOrNull()
+            if (zone != null) {
+                add("DTSTART;TZID=${event.zoneId}:${localStamp(event.start, zone)}")
+                add("DTEND;TZID=${event.zoneId}:${localStamp(event.end, zone)}")
+            } else {
+                // Unknown zone id → fall back to plain UTC instants.
+                add("DTSTART:${utcStamp(event.start)}")
+                add("DTEND:${utcStamp(event.end)}")
+            }
+        }
+        else -> {
+            add("DTSTART:${utcStamp(event.start)}")
+            add("DTEND:${utcStamp(event.end)}")
+        }
+    }
+
+    private fun MutableList<String>.appendAlarm(minutes: Int, summary: String) {
+        add("BEGIN:VALARM")
+        add("ACTION:DISPLAY")
+        add("DESCRIPTION:${escapeText(summary.ifBlank { "Reminder" })}")
+        add("TRIGGER:${triggerValue(minutes)}")
+        add("END:VALARM")
+    }
+
+    private companion object {
+        fun statusValue(status: EventStatus): String = when (status) {
+            EventStatus.Confirmed -> "CONFIRMED"
+            EventStatus.Tentative -> "TENTATIVE"
+            EventStatus.Cancelled -> "CANCELLED"
+        }
+
+        // iCal TRANSP is binary; only an explicitly free event is TRANSPARENT.
+        fun transpValue(availability: Availability): String =
+            if (availability == Availability.Free) "TRANSPARENT" else "OPAQUE"
+
+        // A lead time of 0 fires at start (PT0M); anything positive is "before".
+        fun triggerValue(minutes: Int): String =
+            if (minutes <= 0) "PT0M" else "-PT${minutes}M"
+
+        fun utcStamp(instant: Instant): String =
+            basic(instant.toLocalDateTime(TimeZone.UTC)) + "Z"
+
+        fun localStamp(instant: Instant, zone: TimeZone): String =
+            basic(instant.toLocalDateTime(zone))
+
+        fun utcDate(instant: Instant): String {
+            val dt = instant.toLocalDateTime(TimeZone.UTC)
+            return "%04d%02d%02d".format(dt.year, dt.month.number, dt.day)
+        }
+
+        fun basic(dt: LocalDateTime): String = "%04d%02d%02dT%02d%02d%02d".format(
+            dt.year, dt.month.number, dt.day, dt.hour, dt.minute, dt.second,
+        )
+    }
+}
--- a/app/src/main/java/de/jeanlucmakiola/calendula/ui/calendars/CalendarsScreen.kt
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/ui/calendars/CalendarsScreen.kt
@@ -5,6 +5,8 @@ import android.content.Context
 import android.content.Intent
 import android.provider.Settings
 import androidx.activity.compose.BackHandler
+import androidx.activity.compose.rememberLauncherForActivityResult
+import androidx.activity.result.contract.ActivityResultContracts
 import androidx.compose.foundation.background
 import androidx.compose.foundation.isSystemInDarkTheme
 import androidx.compose.foundation.layout.Arrangement
@@ -30,6 +32,7 @@ import androidx.compose.material.icons.filled.CalendarMonth
 import androidx.compose.material.icons.filled.Close
 import androidx.compose.material.icons.filled.Delete
 import androidx.compose.material.icons.filled.Edit
+import androidx.compose.material.icons.filled.FileDownload
 import androidx.compose.material.icons.filled.OpenInNew
 import androidx.compose.material.icons.filled.Palette
 import androidx.compose.material3.AlertDialog
@@ -77,6 +80,7 @@ import de.jeanlucmakiola.calendula.ui.common.InlineTextField
 import de.jeanlucmakiola.calendula.ui.common.Position
 import de.jeanlucmakiola.calendula.ui.common.pastelize
 import de.jeanlucmakiola.calendula.ui.common.positionOf
+import java.time.LocalDate

 /** Sentinel [editorId] meaning "the editor is composing a new calendar". */
 private const val NEW_CALENDAR_ID = Long.MIN_VALUE
@@ -95,6 +99,7 @@ fun CalendarsScreen(
 ) {
    val calendars by viewModel.calendars.collectAsStateWithLifecycle()
    val error by viewModel.error.collectAsStateWithLifecycle()
+    val backupResult by viewModel.backupResult.collectAsStateWithLifecycle()

    // null = list; NEW_CALENDAR_ID = create; any other id = edit that calendar.
    // [editorSession] bumps on every open so the editor's field state resets for
@@ -131,6 +136,9 @@ fun CalendarsScreen(
            synced = calendars.filterNot { it.isLocal },
            error = error,
            onConsumeError = viewModel::consumeError,
+            backupResult = backupResult,
+            onExportBackup = viewModel::exportBackup,
+            onConsumeBackupResult = viewModel::consumeBackupResult,
            onBack = onBack,
            onAdd = { editorSession++; editorId = NEW_CALENDAR_ID },
            onEdit = { calendar -> editorSession++; editorId = calendar.id },
@@ -144,6 +152,9 @@ private fun CalendarsList(
    synced: List<CalendarSource>,
    error: Boolean,
    onConsumeError: () -> Unit,
+    backupResult: BackupResult?,
+    onExportBackup: (android.net.Uri) -> Unit,
+    onConsumeBackupResult: () -> Unit,
    onBack: () -> Unit,
    onAdd: () -> Unit,
    onEdit: (CalendarSource) -> Unit,
@@ -159,6 +170,31 @@ private fun CalendarsList(
        }
    }

+    // SAF "create document" target for the backup file. The picked Uri is handed
+    // to the VM to stream the .ics into.
+    val createBackup = rememberLauncherForActivityResult(
+        contract = ActivityResultContracts.CreateDocument("text/calendar"),
+    ) { uri -> uri?.let(onExportBackup) }
+
+    val backupFailedText = stringResource(R.string.calendars_backup_failed)
+    LaunchedEffect(backupResult) {
+        when (val r = backupResult) {
+            is BackupResult.Success -> {
+                snackbarHostState.showSnackbar(
+                    context.resources.getQuantityString(
+                        R.plurals.calendars_backup_done, r.eventCount, r.eventCount,
+                    ),
+                )
+                onConsumeBackupResult()
+            }
+            BackupResult.Failure -> {
+                snackbarHostState.showSnackbar(backupFailedText)
+                onConsumeBackupResult()
+            }
+            null -> Unit
+        }
+    }
+
    CollapsingScaffold(
        title = stringResource(R.string.calendars_title),
        onBack = onBack,
@@ -195,6 +231,22 @@ private fun CalendarsList(
            onClick = onAdd,
        )

+        // Backup — local calendars have no sync, so a .ics export is their only
+        // safety net. Offered only when there is something to back up.
+        if (local.isNotEmpty()) {
+            Spacer(Modifier.height(16.dp))
+            SectionHeader(stringResource(R.string.calendars_backup_header))
+            HintText(stringResource(R.string.calendars_backup_hint))
+            GroupedRow(
+                title = stringResource(R.string.calendars_backup_action),
+                position = Position.Alone,
+                leading = { LeadingAvatar(Icons.Default.FileDownload) },
+                onClick = {
+                    runCatching { createBackup.launch("calendula-backup-${LocalDate.now()}.ics") }
+                },
+            )
+        }
+
        Spacer(Modifier.height(16.dp))

        // Synced calendars — read-only, grouped by account, each with a
@@ -429,6 +481,25 @@ private fun AccountHeader(account: String, accountType: String) {
    }
 }

+/** Neutral circular chip carrying an arbitrary icon — matches [AddAvatar]'s shape. */
+@Composable
+private fun LeadingAvatar(icon: ImageVector) {
+    Box(
+        modifier = Modifier
+            .size(40.dp)
+            .clip(CircleShape)
+            .background(MaterialTheme.colorScheme.surfaceContainerHighest),
+        contentAlignment = Alignment.Center,
+    ) {
+        Icon(
+            icon,
+            contentDescription = null,
+            tint = MaterialTheme.colorScheme.onSurfaceVariant,
+            modifier = Modifier.size(22.dp),
+        )
+    }
+}
+
 /** Neutral circular chip with a "+" — the leading icon for add-actions. */
@Composable
 private fun AddAvatar() {
--- a/app/src/main/java/de/jeanlucmakiola/calendula/ui/calendars/CalendarsViewModel.kt
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/ui/calendars/CalendarsViewModel.kt
@@ -1,11 +1,14 @@
 package de.jeanlucmakiola.calendula.ui.calendars

+import android.net.Uri
 import androidx.lifecycle.ViewModel
 import androidx.lifecycle.viewModelScope
 import dagger.hilt.android.lifecycle.HiltViewModel
 import de.jeanlucmakiola.calendula.data.calendar.CalendarRepository
 import de.jeanlucmakiola.calendula.data.di.IoDispatcher
+import de.jeanlucmakiola.calendula.data.ics.IcsExporter
 import de.jeanlucmakiola.calendula.domain.CalendarSource
+import de.jeanlucmakiola.calendula.domain.ics.IcsWriter
 import kotlinx.coroutines.CoroutineDispatcher
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.SharingStarted
@@ -15,7 +18,9 @@ import kotlinx.coroutines.flow.catch
 import kotlinx.coroutines.flow.flowOn
 import kotlinx.coroutines.flow.stateIn
 import kotlinx.coroutines.launch
+import kotlinx.coroutines.withContext
 import kotlin.coroutines.cancellation.CancellationException
+import kotlin.time.Clock
 import javax.inject.Inject

 /**
@@ -27,6 +32,7 @@ import javax.inject.Inject
@HiltViewModel
 class CalendarsViewModel @Inject constructor(
    private val repository: CalendarRepository,
+    private val icsExporter: IcsExporter,
    @IoDispatcher private val io: CoroutineDispatcher,
 ) : ViewModel() {

@@ -45,6 +51,36 @@ class CalendarsViewModel @Inject constructor(

    fun consumeError() { _error.value = false }

+    private val _backupResult = MutableStateFlow<BackupResult?>(null)
+    val backupResult: StateFlow<BackupResult?> = _backupResult.asStateFlow()
+
+    fun consumeBackupResult() { _backupResult.value = null }
+
+    /**
+     * Serialise every event of the writable local calendars into the chosen SAF
+     * document [uri] as one `VCALENDAR`. Result (event count, or failure) lands
+     * in [backupResult] for a one-shot message.
+     */
+    fun exportBackup(uri: Uri) {
+        viewModelScope.launch {
+            _backupResult.value = try {
+                val count = withContext(io) {
+                    val events = repository.exportEvents()
+                    icsExporter.writeDocument(
+                        uri = uri,
+                        content = IcsWriter().writeCalendar(events, Clock.System.now()),
+                    )
+                    events.size
+                }
+                BackupResult.Success(count)
+            } catch (e: CancellationException) {
+                throw e
+            } catch (e: Exception) {
+                BackupResult.Failure
+            }
+        }
+    }
+
    fun createCalendar(displayName: String, color: Int, description: String?) = write {
        repository.createLocalCalendar(displayName, color, description)
    }
@@ -69,3 +105,9 @@ class CalendarsViewModel @Inject constructor(
        }
    }
 }
+
+/** Outcome of a whole-calendar backup, surfaced once to the screen. */
+sealed interface BackupResult {
+    data class Success(val eventCount: Int) : BackupResult
+    data object Failure : BackupResult
+}
--- a/app/src/main/java/de/jeanlucmakiola/calendula/ui/detail/EventDetailScreen.kt
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/ui/detail/EventDetailScreen.kt
@@ -41,6 +41,7 @@ import androidx.compose.material.icons.filled.Place
 import androidx.compose.material.icons.filled.Public
 import androidx.compose.material.icons.filled.Repeat
 import androidx.compose.material.icons.filled.Schedule
+import androidx.compose.material.icons.filled.Share
 import androidx.compose.material3.AlertDialog
 import androidx.compose.material3.ExperimentalMaterial3Api
 import androidx.compose.material3.Icon
@@ -59,6 +60,7 @@ import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
+import androidx.compose.runtime.rememberCoroutineScope
 import androidx.compose.runtime.saveable.rememberSaveable
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
@@ -96,6 +98,7 @@ import de.jeanlucmakiola.calendula.ui.common.currentLocale
 import de.jeanlucmakiola.calendula.ui.common.pastelize
 import de.jeanlucmakiola.calendula.ui.common.recurrenceText
 import de.jeanlucmakiola.calendula.ui.common.reminderLeadTimeLabel
+import kotlinx.coroutines.launch
 import kotlinx.datetime.TimeZone
 import java.time.ZoneId
 import java.time.format.DateTimeFormatter
@@ -132,9 +135,30 @@ fun EventDetailScreen(
    BackHandler(onBack = onBack)

    val context = LocalContext.current
+    val scope = rememberCoroutineScope()
    val snackbarHostState = remember { SnackbarHostState() }
    var showDeleteDialog by rememberSaveable { mutableStateOf(false) }

+    // Sharing is read-only, so it needs no WRITE_CALENDAR upgrade. The VM stages
+    // an .ics in the cache and hands back a content Uri for the chooser.
+    val shareFailedMessage = stringResource(R.string.event_share_failed)
+    val shareChooserTitle = stringResource(R.string.event_share_chooser_title)
+    val onShareClick = {
+        scope.launch {
+            val uri = viewModel.shareUri()
+            val sent = uri != null && runCatching {
+                val send = Intent(Intent.ACTION_SEND).apply {
+                    type = "text/calendar"
+                    putExtra(Intent.EXTRA_STREAM, uri)
+                    addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION)
+                }
+                context.startActivity(Intent.createChooser(send, shareChooserTitle))
+            }.isSuccess
+            if (!sent) snackbarHostState.showSnackbar(shareFailedMessage)
+        }
+        Unit
+    }
+
    // v1.0 installs only hold READ_CALENDAR; the first write asks for the
    // upgrade in place. Granting continues straight into the tapped action.
    var pendingEdit by remember { mutableStateOf(false) }
@@ -203,9 +227,18 @@ fun EventDetailScreen(
                    }
                },
                actions = {
-                    // Only writable calendars get actions — WebCal subscriptions,
-                    // birthday calendars etc. are read-only at the provider level.
                    val s = state
+                    // Share works for any loaded event — it only reads the event.
+                    if (s is EventDetailUiState.Success) {
+                        IconButton(onClick = onShareClick) {
+                            Icon(
+                                imageVector = Icons.Default.Share,
+                                contentDescription = stringResource(R.string.event_detail_share),
+                            )
+                        }
+                    }
+                    // Edit/delete need a writable calendar — WebCal subscriptions,
+                    // birthday calendars etc. are read-only at the provider level.
                    if (s is EventDetailUiState.Success && s.canModify) {
                        IconButton(
                            onClick = onEditClick,
--- a/app/src/main/java/de/jeanlucmakiola/calendula/ui/detail/EventDetailViewModel.kt
+++ b/app/src/main/java/de/jeanlucmakiola/calendula/ui/detail/EventDetailViewModel.kt
@@ -1,14 +1,20 @@
 package de.jeanlucmakiola.calendula.ui.detail

+import android.net.Uri
 import androidx.lifecycle.ViewModel
 import androidx.lifecycle.viewModelScope
 import dagger.hilt.android.lifecycle.HiltViewModel
 import de.jeanlucmakiola.calendula.data.calendar.CalendarRepository
 import de.jeanlucmakiola.calendula.data.calendar.NoSuchEventException
 import de.jeanlucmakiola.calendula.data.di.IoDispatcher
+import de.jeanlucmakiola.calendula.data.ics.IcsExporter
 import de.jeanlucmakiola.calendula.domain.FailureReason
 import de.jeanlucmakiola.calendula.domain.RecurringWriteScope
+import de.jeanlucmakiola.calendula.domain.ics.IcsWriter
+import de.jeanlucmakiola.calendula.domain.ics.toShareIcsEvent
 import kotlinx.coroutines.CoroutineDispatcher
+import kotlinx.coroutines.withContext
+import kotlin.time.Clock
 import kotlinx.coroutines.ExperimentalCoroutinesApi
 import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.SharingStarted
@@ -34,6 +40,7 @@ import javax.inject.Inject
@HiltViewModel
 class EventDetailViewModel @Inject constructor(
    private val repository: CalendarRepository,
+    private val icsExporter: IcsExporter,
    @IoDispatcher private val io: CoroutineDispatcher,
 ) : ViewModel() {

@@ -113,6 +120,24 @@ class EventDetailViewModel @Inject constructor(
        _deleteState.value = DeleteUiState.Idle
    }

+    /**
+     * Serialise the open event to a `.ics` cache file and return a shareable
+     * content Uri (for an ACTION_SEND), or null when nothing is loaded or the
+     * write fails. The event is exported as a one-off (see [toShareIcsEvent]).
+     */
+    suspend fun shareUri(): Uri? {
+        val detail = (state.value as? EventDetailUiState.Success)?.detail ?: return null
+        return runCatching {
+            withContext(io) {
+                val ics = IcsWriter().writeCalendar(
+                    events = listOf(detail.toShareIcsEvent()),
+                    dtStamp = Clock.System.now(),
+                )
+                icsExporter.stageShareFile(shareFileName(detail.instance.title), ics)
+            }
+        }.getOrNull()
+    }
+
    private suspend fun loadDetail(target: Target): EventDetailUiState = try {
        val detail = repository.eventDetail(target.eventId)
        // The Events row holds the series start; replace it with this
@@ -143,3 +168,13 @@ class EventDetailViewModel @Inject constructor(
    /** A tapped occurrence: the series [eventId] plus this occurrence's own times. */
    private data class Target(val eventId: Long, val beginMillis: Long, val endMillis: Long)
 }
+
+/** A filesystem-safe `.ics` file name from an event title (or a fallback). */
+private fun shareFileName(title: String): String {
+    val base = title.trim()
+        .replace(Regex("""[^\p{L}\p{Nd} _-]"""), "")
+        .replace(' ', '_')
+        .take(40)
+        .ifBlank { "event" }
+    return "$base.ics"
+}
--- a/app/src/main/res/values-de/strings.xml
+++ b/app/src/main/res/values-de/strings.xml
@@ -47,6 +47,9 @@
    <string name="event_detail_back">Zurück</string>
    <string name="event_detail_edit">Bearbeiten</string>
    <string name="event_detail_delete">Löschen</string>
+    <string name="event_detail_share">Teilen</string>
+    <string name="event_share_chooser_title">Termin teilen</string>
+    <string name="event_share_failed">Termin konnte nicht geteilt werden.</string>
    <string name="event_delete_title">Termin löschen?</string>
    <string name="event_delete_body">Der Termin wird aus deinem Kalender und von allen synchronisierten Geräten entfernt.</string>
    <string name="event_delete_recurring_title">Wiederkehrenden Termin löschen</string>
@@ -297,4 +300,13 @@
    <string name="calendars_delete_confirm_title">Kalender löschen?</string>
    <string name="calendars_delete_confirm_message">\"%1$s\" und alle zugehörigen Termine werden dauerhaft von diesem Gerät entfernt.</string>
    <string name="calendars_write_error">Änderung konnte nicht gespeichert werden.</string>
+    <!-- Backup (whole-calendar .ics export) -->
+    <string name="calendars_backup_header">Sicherung</string>
+    <string name="calendars_backup_hint">Lokale Kalender werden nirgends synchronisiert – exportiere sie als .ics-Datei, um eine Kopie zu behalten.</string>
+    <string name="calendars_backup_action">Als .ics-Datei exportieren</string>
+    <string name="calendars_backup_failed">Sicherung konnte nicht exportiert werden.</string>
+    <plurals name="calendars_backup_done">
+        <item quantity="one">%d Termin exportiert.</item>
+        <item quantity="other">%d Termine exportiert.</item>
+    </plurals>
 </resources>
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -48,6 +48,9 @@
    <string name="event_detail_back">Back</string>
    <string name="event_detail_edit">Edit</string>
    <string name="event_detail_delete">Delete</string>
+    <string name="event_detail_share">Share</string>
+    <string name="event_share_chooser_title">Share event</string>
+    <string name="event_share_failed">Couldn\'t share this event.</string>
    <string name="event_delete_title">Delete event?</string>
    <string name="event_delete_body">The event is removed from your calendar and from every device it syncs to.</string>
    <string name="event_delete_recurring_title">Delete recurring event</string>
@@ -294,6 +297,15 @@
    <string name="calendars_delete_confirm_title">Delete calendar?</string>
    <string name="calendars_delete_confirm_message">\"%1$s\" and all of its events will be permanently removed from this device.</string>
    <string name="calendars_write_error">Couldn\'t save the change.</string>
+    <!-- Backup (whole-calendar .ics export) -->
+    <string name="calendars_backup_header">Backup</string>
+    <string name="calendars_backup_hint">Local calendars aren\'t synced anywhere, so export them to an .ics file to keep a copy.</string>
+    <string name="calendars_backup_action">Export as .ics file</string>
+    <string name="calendars_backup_failed">Couldn\'t export the backup.</string>
+    <plurals name="calendars_backup_done">
+        <item quantity="one">Exported %d event.</item>
+        <item quantity="other">Exported %d events.</item>
+    </plurals>
    <!-- Launcher long-press shortcuts -->
    <string name="shortcut_new_event_short">New event</string>
    <string name="shortcut_new_event_long">Create a new event</string>
--- a/app/src/main/res/xml/file_paths.xml
+++ b/app/src/main/res/xml/file_paths.xml
@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Exposes the cache subdirectory where IcsExporter stages files for sharing. -->
+<paths>
+    <cache-path
+        name="shared_ics"
+        path="shared_ics/" />
+</paths>
--- a/app/src/test/java/de/jeanlucmakiola/calendula/data/calendar/FakeCalendarDataSource.kt
+++ b/app/src/test/java/de/jeanlucmakiola/calendula/data/calendar/FakeCalendarDataSource.kt
@@ -5,6 +5,7 @@ import de.jeanlucmakiola.calendula.domain.EventColorOption
 import de.jeanlucmakiola.calendula.domain.EventDetail
 import de.jeanlucmakiola.calendula.domain.EventForm
 import de.jeanlucmakiola.calendula.domain.EventInstance
+import de.jeanlucmakiola.calendula.domain.ics.IcsEvent

 /**
 * Test-only fake. Tunable via the three `var` properties; `tick()` simulates
@@ -16,6 +17,7 @@ internal class FakeCalendarDataSource : CalendarDataSource {
    var instancesResult: (Long, Long) -> List<EventInstance> = { _, _ -> emptyList() }
    var eventDetailResult: (Long) -> EventDetail? = { null }
    var eventColorPaletteResult: (Long) -> List<EventColorOption> = { emptyList() }
+    var exportableEventsResult: List<IcsEvent> = emptyList()
    /** Set to make the next write call throw. */
    var writeError: Exception? = null
    /** Id returned by the next [insertEvent]. */
@@ -49,6 +51,7 @@ internal class FakeCalendarDataSource : CalendarDataSource {
    override fun eventDetail(eventId: Long): EventDetail? = eventDetailResult(eventId)
    override fun eventColorPalette(calendarId: Long): List<EventColorOption> =
        eventColorPaletteResult(calendarId)
+    override fun exportableEvents(): List<IcsEvent> = exportableEventsResult

    override fun createLocalCalendar(displayName: String, color: Int, description: String?): Long {
        writeError?.let { throw it }
--- a/app/src/test/java/de/jeanlucmakiola/calendula/data/calendar/IcsExportMapperTest.kt
+++ b/app/src/test/java/de/jeanlucmakiola/calendula/data/calendar/IcsExportMapperTest.kt
@@ -0,0 +1,80 @@
+package de.jeanlucmakiola.calendula.data.calendar
+
+import android.provider.CalendarContract
+import com.google.common.truth.Truth.assertThat
+import de.jeanlucmakiola.calendula.domain.EventStatus
+import org.junit.jupiter.api.Test
+
+class IcsExportMapperTest {
+
+    @Test
+    fun `parses the duration forms Calendula writes plus general ones`() {
+        assertThat(parseRfc2445DurationMillis("P1D")).isEqualTo(86_400_000L)
+        assertThat(parseRfc2445DurationMillis("P3600S")).isEqualTo(3_600_000L)
+        assertThat(parseRfc2445DurationMillis("PT1H30M")).isEqualTo(5_400_000L)
+        assertThat(parseRfc2445DurationMillis("P1W")).isEqualTo(604_800_000L)
+        assertThat(parseRfc2445DurationMillis(null)).isEqualTo(0L)
+        assertThat(parseRfc2445DurationMillis("nonsense")).isEqualTo(0L)
+    }
+
+    @Test
+    fun `timed one-off row maps with its DTEND and kept UID`() {
+        val reader = MapColumnReader(
+            EventExportProjection.IDX_ID to 42L,
+            EventExportProjection.IDX_UID to "abc@host",
+            EventExportProjection.IDX_TITLE to "Standup",
+            EventExportProjection.IDX_DTSTART to 1_000_000L,
+            EventExportProjection.IDX_DTEND to 1_900_000L,
+            EventExportProjection.IDX_ALL_DAY to 0,
+            EventExportProjection.IDX_EVENT_TIMEZONE to "Europe/Berlin",
+            EventExportProjection.IDX_AVAILABILITY to CalendarContract.Events.AVAILABILITY_BUSY,
+        )
+
+        val event = reader.toIcsEvent(reminderMinutes = listOf(10), calendarName = "Personal")
+
+        assertThat(event.uid).isEqualTo("abc@host")
+        assertThat(event.summary).isEqualTo("Standup")
+        assertThat(event.start.toEpochMilliseconds()).isEqualTo(1_000_000L)
+        assertThat(event.end.toEpochMilliseconds()).isEqualTo(1_900_000L)
+        assertThat(event.isAllDay).isFalse()
+        assertThat(event.recurrenceRule).isNull()
+        assertThat(event.reminderMinutes).containsExactly(10)
+        assertThat(event.calendarName).isEqualTo("Personal")
+        assertThat(event.status).isEqualTo(EventStatus.Confirmed)
+    }
+
+    @Test
+    fun `recurring row without DTEND reconstructs end from DURATION`() {
+        val reader = MapColumnReader(
+            EventExportProjection.IDX_ID to 7L,
+            // No UID column → synthesised stably from id + dtstart.
+            EventExportProjection.IDX_TITLE to "Weekly",
+            EventExportProjection.IDX_DTSTART to 1_000_000L,
+            // DTEND absent (null); DURATION carries the length.
+            EventExportProjection.IDX_DURATION to "P3600S",
+            EventExportProjection.IDX_ALL_DAY to 0,
+            EventExportProjection.IDX_RRULE to "FREQ=WEEKLY",
+            EventExportProjection.IDX_EVENT_TIMEZONE to "UTC",
+        )
+
+        val event = reader.toIcsEvent(reminderMinutes = emptyList(), calendarName = null)
+
+        assertThat(event.uid).isEqualTo("7-1000000@calendula")
+        assertThat(event.recurrenceRule).isEqualTo("FREQ=WEEKLY")
+        assertThat(event.end.toEpochMilliseconds()).isEqualTo(1_000_000L + 3_600_000L)
+    }
+
+    @Test
+    fun `all-day flag is carried through`() {
+        val reader = MapColumnReader(
+            EventExportProjection.IDX_ID to 1L,
+            EventExportProjection.IDX_TITLE to "Holiday",
+            EventExportProjection.IDX_DTSTART to 0L,
+            EventExportProjection.IDX_DTEND to 86_400_000L,
+            EventExportProjection.IDX_ALL_DAY to 1,
+            EventExportProjection.IDX_EVENT_TIMEZONE to "UTC",
+        )
+
+        assertThat(reader.toIcsEvent(emptyList(), null).isAllDay).isTrue()
+    }
+}
--- a/app/src/test/java/de/jeanlucmakiola/calendula/domain/ics/IcsTextTest.kt
+++ b/app/src/test/java/de/jeanlucmakiola/calendula/domain/ics/IcsTextTest.kt
@@ -0,0 +1,57 @@
+package de.jeanlucmakiola.calendula.domain.ics
+
+import com.google.common.truth.Truth.assertThat
+import org.junit.jupiter.api.Test
+
+class IcsTextTest {
+
+    @Test
+    fun `escapes backslash semicolon comma and newline`() {
+        assertThat(escapeText("a\\b;c,d\ne"))
+            .isEqualTo("a\\\\b\\;c\\,d\\ne")
+    }
+
+    @Test
+    fun `backslash is escaped before its escape markers, not after`() {
+        // A single backslash must become exactly one escaped backslash, not
+        // accidentally combine with a following separator.
+        assertThat(escapeText("\\;")).isEqualTo("\\\\\\;")
+    }
+
+    @Test
+    fun `short line is returned unfolded`() {
+        val line = "SUMMARY:short"
+        assertThat(foldLine(line)).isEqualTo(line)
+    }
+
+    @Test
+    fun `long line folds into physical lines of at most 75 octets`() {
+        val line = "DESCRIPTION:" + "x".repeat(300)
+        val folded = foldLine(line)
+
+        val physical = folded.split(ICS_CRLF)
+        assertThat(physical.size).isGreaterThan(1)
+        physical.forEach { assertThat(it.toByteArray(Charsets.UTF_8).size).isAtMost(75) }
+        // Every continuation line begins with the single folding space.
+        physical.drop(1).forEach { assertThat(it).startsWith(" ") }
+    }
+
+    @Test
+    fun `unfolding a folded line restores the original`() {
+        val line = "DESCRIPTION:" + "abcdefg ".repeat(40).trim()
+        val unfolded = foldLine(line).replace(ICS_CRLF + " ", "")
+        assertThat(unfolded).isEqualTo(line)
+    }
+
+    @Test
+    fun `folding never splits a multi-byte character`() {
+        // 100 emoji (4 UTF-8 octets each) — a naive byte split would corrupt one.
+        val line = "X-NOTE:" + "😀".repeat(100)
+        val folded = foldLine(line)
+        // The reassembled content must still decode to the same string.
+        assertThat(folded.replace(ICS_CRLF + " ", "")).isEqualTo(line)
+        folded.split(ICS_CRLF).forEach {
+            assertThat(it.toByteArray(Charsets.UTF_8).size).isAtMost(75)
+        }
+    }
+}
--- a/app/src/test/java/de/jeanlucmakiola/calendula/domain/ics/IcsWriterTest.kt
+++ b/app/src/test/java/de/jeanlucmakiola/calendula/domain/ics/IcsWriterTest.kt
@@ -0,0 +1,152 @@
+package de.jeanlucmakiola.calendula.domain.ics
+
+import com.google.common.truth.Truth.assertThat
+import de.jeanlucmakiola.calendula.domain.Availability
+import de.jeanlucmakiola.calendula.domain.EventStatus
+import kotlinx.datetime.LocalDate
+import kotlinx.datetime.LocalDateTime
+import kotlinx.datetime.TimeZone
+import kotlinx.datetime.atStartOfDayIn
+import kotlinx.datetime.toInstant
+import org.junit.jupiter.api.Test
+import kotlin.time.Instant
+
+class IcsWriterTest {
+
+    private val writer = IcsWriter(prodId = "-//Test//Test//EN")
+    private val stamp = LocalDateTime(2026, 6, 18, 9, 30, 0).toInstant(TimeZone.UTC)
+
+    private fun lines(events: List<IcsEvent>): List<String> =
+        writer.writeCalendar(events, stamp).split(ICS_CRLF)
+
+    private fun instantUtc(y: Int, mo: Int, d: Int, h: Int, mi: Int): Instant =
+        LocalDateTime(y, mo, d, h, mi, 0).toInstant(TimeZone.UTC)
+
+    @Test
+    fun `calendar is wrapped with the required header and CRLF endings`() {
+        val out = writer.writeCalendar(emptyList(), stamp)
+        assertThat(out).startsWith("BEGIN:VCALENDAR\r\n")
+        assertThat(out).endsWith("END:VCALENDAR\r\n")
+        assertThat(out).contains("VERSION:2.0\r\n")
+        assertThat(out).contains("PRODID:-//Test//Test//EN\r\n")
+    }
+
+    @Test
+    fun `timed one-off event writes UTC instants with a Z suffix`() {
+        val event = IcsEvent(
+            uid = "u1@calendula",
+            summary = "Standup",
+            start = instantUtc(2026, 6, 18, 13, 0),
+            end = instantUtc(2026, 6, 18, 13, 30),
+            isAllDay = false,
+            zoneId = "Europe/Berlin",
+        )
+        val l = lines(listOf(event))
+        assertThat(l).contains("DTSTART:20260618T130000Z")
+        assertThat(l).contains("DTEND:20260618T133000Z")
+        assertThat(l).contains("UID:u1@calendula")
+        assertThat(l).contains("STATUS:CONFIRMED")
+        assertThat(l).contains("TRANSP:OPAQUE")
+    }
+
+    @Test
+    fun `recurring timed event anchors to wall-clock with TZID`() {
+        // 13:00 UTC == 15:00 Berlin in summer; the series must read 15:00 local.
+        val event = IcsEvent(
+            uid = "u2@calendula",
+            summary = "Weekly",
+            start = instantUtc(2026, 6, 18, 13, 0),
+            end = instantUtc(2026, 6, 18, 14, 0),
+            isAllDay = false,
+            zoneId = "Europe/Berlin",
+            recurrenceRule = "FREQ=WEEKLY",
+        )
+        val l = lines(listOf(event))
+        assertThat(l).contains("DTSTART;TZID=Europe/Berlin:20260618T150000")
+        assertThat(l).contains("DTEND;TZID=Europe/Berlin:20260618T160000")
+        assertThat(l).contains("RRULE:FREQ=WEEKLY")
+    }
+
+    @Test
+    fun `recurring event with an unknown zone falls back to UTC instants`() {
+        val event = IcsEvent(
+            uid = "u3@calendula",
+            summary = "Weekly",
+            start = instantUtc(2026, 6, 18, 13, 0),
+            end = instantUtc(2026, 6, 18, 14, 0),
+            isAllDay = false,
+            zoneId = "Mars/Olympus",
+            recurrenceRule = "FREQ=WEEKLY",
+        )
+        val l = lines(listOf(event))
+        assertThat(l).contains("DTSTART:20260618T130000Z")
+        assertThat(l).contains("DTEND:20260618T140000Z")
+    }
+
+    @Test
+    fun `all-day event writes exclusive DATE values without a zone`() {
+        val start = LocalDate(2026, 6, 18).atStartOfDayIn(TimeZone.UTC)
+        val end = LocalDate(2026, 6, 19).atStartOfDayIn(TimeZone.UTC)
+        val event = IcsEvent(
+            uid = "u4@calendula",
+            summary = "Holiday",
+            start = start,
+            end = end,
+            isAllDay = true,
+            zoneId = "UTC",
+        )
+        val l = lines(listOf(event))
+        assertThat(l).contains("DTSTART;VALUE=DATE:20260618")
+        assertThat(l).contains("DTEND;VALUE=DATE:20260619")
+    }
+
+    @Test
+    fun `reminders become VALARM blocks with before-start triggers`() {
+        val event = IcsEvent(
+            uid = "u5@calendula",
+            summary = "Meeting",
+            start = instantUtc(2026, 6, 18, 13, 0),
+            end = instantUtc(2026, 6, 18, 14, 0),
+            isAllDay = false,
+            zoneId = "UTC",
+            reminderMinutes = listOf(15, 0, 15), // duplicate is dropped
+        )
+        val out = writer.writeCalendar(listOf(event), stamp)
+        val l = out.split(ICS_CRLF)
+        assertThat(l.count { it == "BEGIN:VALARM" }).isEqualTo(2)
+        assertThat(l).contains("TRIGGER:-PT15M")
+        assertThat(l).contains("TRIGGER:PT0M")
+        assertThat(l).contains("ACTION:DISPLAY")
+    }
+
+    @Test
+    fun `text fields and the calendar name are escaped`() {
+        val event = IcsEvent(
+            uid = "u6@calendula",
+            summary = "Lunch; with, notes",
+            start = instantUtc(2026, 6, 18, 13, 0),
+            end = instantUtc(2026, 6, 18, 14, 0),
+            isAllDay = false,
+            zoneId = "UTC",
+            location = "Cafe\\Bar",
+            availability = Availability.Free,
+            status = EventStatus.Tentative,
+            calendarName = "Work, Personal",
+        )
+        val l = lines(listOf(event))
+        assertThat(l).contains("SUMMARY:Lunch\\; with\\, notes")
+        assertThat(l).contains("LOCATION:Cafe\\\\Bar")
+        assertThat(l).contains("STATUS:TENTATIVE")
+        assertThat(l).contains("TRANSP:TRANSPARENT")
+        assertThat(l).contains("X-CALENDULA-CALENDAR:Work\\, Personal")
+    }
+
+    @Test
+    fun `existing uid is kept and a missing one is synthesised stably`() {
+        assertThat(deriveIcsUid("real-uid@host", 7, 1000)).isEqualTo("real-uid@host")
+        assertThat(deriveIcsUid(null, 7, 1000)).isEqualTo("7-1000@calendula")
+        assertThat(deriveIcsUid("  ", 7, 1000)).isEqualTo("7-1000@calendula")
+        // Stable across calls — a re-export of the same row yields the same UID.
+        assertThat(deriveIcsUid(null, 7, 1000)).isEqualTo(deriveIcsUid(null, 7, 1000))
+    }
+}
--- a/docs/superpowers/plans/2026-06-18-05-ics-export.md
+++ b/docs/superpowers/plans/2026-06-18-05-ics-export.md
@@ -0,0 +1,150 @@
+# Calendula - Plan 05: ICS Export (v2.7, Branch 1 von 2)
+
+> **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking.
+
+**Goal:** Die Schreib-Hälfte des `.ics`-Themas. Calendula serialisiert eigene
+Events nach RFC 5545 — als Einzel-Event (Share-Sheet) und als
+Ganz-Kalender-Backup (SAF-Datei). Damit existiert für gerätelokale Kalender
+(`ACCOUNT_TYPE_LOCAL`) zum ersten Mal ein Backup; ohne Sync ist ein verlorenes
+Gerät sonst Totalverlust. Diese Branch baut **nur Export**; Import
+(Parser, Restore, Open-into-form) folgt in Branch 2 (`feat/ics-import`),
+beide landen zusammen in **einem** Release v2.7.0 — es gibt also keine
+Zwischenversion, die UIDs schreibt, ohne sie je zu lesen.
+`./gradlew lint test assembleDebug` bleibt grün; Release erst nach
+On-Device-Review (gemeinsam mit Branch 2).
+
+**Architecture:** Neue reine-Kotlin-Engine `domain/ics/` — kein
+`CalendarContract`, keine Android-Deps, voll JVM-testbar. Kern ist
+`IcsWriter` (nimmt eine Liste eigener Event-Modelle + Kalender-Metadaten,
+gibt einen `VCALENDAR`-String zurück). Keine ICS-Library: wir bleiben auf
+`kotlinx-datetime` (kein `java.time`-Desugaring) und hand-rollen wie schon
+bei RRULE in `Recurrence.kt`. Das Schreiben in eine SAF-Datei / das
+Share-Intent liegt in einer dünnen Android-Schicht
+(`data/ics/IcsExporter` o. ä.), die Provider-Reads → Domain-Modelle →
+`IcsWriter` → `OutputStream` verdrahtet.
+
+**Recherche-Befunde (Codebase, 2026-06-18):**
+
+1. **Keine ICS-Library, kein `java.time`-Desugaring** — Stack ist
+   `kotlinx-datetime` + `kotlin.time.Instant`. RRULE wird bereits in
+   `domain/Recurrence.kt` von Hand geparst/gerendert (inkl. der sorgfältigen
+   `UNTIL`/DST-Korrektur). `IcsWriter` reiht sich in genau diese Kultur ein und
+   nutzt `SimpleRecurrence.toRRule()` direkt.
+2. **Kein UID-Handling.** `Events.UID_2445` wird heute nirgends gelesen oder
+   geschrieben. Für idempotenten Restore in Branch 2 muss Import auf UID
+   matchen — ohne UID verdoppelt ein erneuter Import alles. Darum schreibt
+   **diese** Branch bereits UID bei jedem Insert (Vorarbeit; ohne Import noch
+   unsichtbar, zahlt sich erst in Branch 2 aus — beide im selben Release).
+3. **Zeitzonen werden gespeichert, aber nie vom Nutzer gewählt.** Lesen/Anzeige:
+   `EVENT_TIMEZONE` landet in `EventDetail.eventTimezone`, das Detail zeigt ein
+   Fremdzonen-Label nur bei Abweichung vom Gerät (`foreignTimeZoneLabel`).
+   Schreiben (`EventWriteMapper.toWriteTimes`): all-day → UTC-Mitternachten,
+   `EVENT_TIMEZONE="UTC"`; getimt → `EVENT_TIMEZONE = zone.id`, und alle Caller
+   übergeben `currentSystemDefault()` (kein Zonen-Feld im Formular). Jedes selbst
+   erstellte Event trägt also die Gerätezone zum Erstellzeitpunkt; eingesynkte
+   Events behalten ihre Originalzone.
+
+**Leitentscheidungen:**
+
+1. **Zeitzonen-Regel beim Schreiben (fallbasiert):**
+   - **All-day** → `DTSTART;VALUE=DATE:YYYYMMDD`, `DTEND` exklusiv
+     (Tag-danach). Keine Zone — trivial korrekt.
+   - **Getimt, nicht wiederkehrend** → UTC-Instant `…T…Z`. Ein Instant ist ein
+     Instant; die Anzeige rechnet beim Import wieder in die Gerätezone. Verlustfrei.
+   - **Getimt, wiederkehrend** → `DTSTART;TZID=<EVENT_TIMEZONE>:<lokale Wandzeit>`.
+     Eine Serie muss an der **Wandzeit** verankert sein, sonst driftet ein
+     „wöchentlich 9 Uhr" über die nächste DST-Grenze um eine Stunde. Die Zone
+     liegt bereits in `EVENT_TIMEZONE` vor; die lokale Wandzeit ist eine
+     `kotlinx-datetime`-Konversion (Instant → LocalDateTime in der Zone).
+   - **`VTIMEZONE`-Blöcke werden bewusst NICHT emittiert.** Beim eigenen
+     Round-Trip löst Branch-2-Import `TZID` gegen die OS-tz-Datenbank auf
+     (`kotlinx-datetime`/`java.time` kennen jede IANA-Id). Technisch nicht
+     RFC-konform; einziger Preis sind strenge Fremd-Parser ohne tz-DB — als
+     bekannte Lücke dokumentiert (Skip-and-report-Territorium des Imports),
+     kein Blocker. Voll-`VTIMEZONE` ist „später, falls nötig".
+2. **UID bei jedem Insert.** `insertEvent` schreibt fortan `Events.UID_2445`
+   (z. B. `<random-uuid>@calendula`). Bestehende Events ohne UID exportieren
+   wir mit einer **deterministischen, stabilen** Fallback-UID, abgeleitet aus
+   `event-id + DTSTART` (`<id>-<dtstart>@calendula`), damit derselbe Bestand
+   über mehrere Backups dieselbe UID behält und Branch-2-Restore nicht
+   verdoppelt. Bestehende Rows werden **nicht** rückwirkend gestempelt
+   (kein Migrations-Sweep über fremde Kalender).
+3. **Manueller Export, kein Background.** Backup via
+   `ACTION_CREATE_DOCUMENT` (SAF, MIME `text/calendar`, Default-Name
+   `calendula-backup-<datum>.ics`); Einzel-Event-Share via `ACTION_SEND` mit
+   einem `FileProvider`-Cache-File (`text/calendar`). Kein WorkManager, kein
+   geplantes/automatisches Backup (passt zum „kein Hintergrunddienst"-Ethos;
+   Auto-Backup bleibt explizit Roadmap-`later`).
+4. **Backup-Layout: eine kombinierte `VCALENDAR`-Datei** über alle
+   gerätelokalen (beschreibbaren) Kalender. Pro Event ein `VEVENT`; die
+   Kalender-Zugehörigkeit reist als `X-WR-CALNAME` / `CATEGORIES` o. ä. mit,
+   damit Branch-2-Restore wieder auffächern oder per Ziel-Picker einsortieren
+   kann. Eine Datei ist einfacher zu teilen/abzulegen als n Dateien.
+   *Offen, vor dem Backup-Task zu fixieren:* exaktes Property fürs
+   Kalender-Mapping (`X-WR-CALNAME` pro `VCALENDAR` erlaubt nur einen Namen;
+   für mehrere Kalender in einer Datei brauchen wir ein Pro-`VEVENT`-Property
+   wie `X-CALENDULA-CALENDAR` oder `CATEGORIES`).
+5. **Feldumfang = was Calendula modelliert.** `IcsWriter` serialisiert genau
+   die gelesenen Felder: `SUMMARY`, `DTSTART`/`DTEND` (Regel #1),
+   `LOCATION`, `DESCRIPTION`, `RRULE` (über `toRRule`), `VALARM` aus den
+   Remindern (DISPLAY, `TRIGGER` = `-PT<min>M`), `STATUS`
+   (CONFIRMED/TENTATIVE/CANCELLED), `TRANSP` (Free→TRANSPARENT/Busy→OPAQUE),
+   `UID`, `DTSTAMP`. Felder ohne sauberes Modell (Attendees, RECURRENCE-ID-
+   Ausnahmen) bleiben **vorerst weg** — Export erzeugt nichts, was Import in
+   Branch 2 nicht auch wieder lesen kann.
+6. **Korrekte RFC-5545-Mechanik:** Zeilen-Folding bei >75 Oktett (CRLF +
+   Space-Fortsetzung), Text-Escaping (`\` `;` `,` `\n`), CRLF-Zeilenenden,
+   `PRODID`/`VERSION:2.0`-Header. Eine reine, einzeln getestete Hilfsschicht
+   (`IcsLine`/`fold`/`escapeText`), nicht ad hoc im Writer verstreut.
+
+---
+
+## Tasks
+
+**Domain-Engine (`domain/ics/`, reine Kotlin, JVM-Tests):**
+- [x] `IcsText`: `escapeText`, `foldLine` (75-Oktett, CRLF+Space) + Test
+      (`IcsTextTest`). Wert-Helfer (Instant→`…T…Z`, Wandzeit→`TZID`,
+      LocalDate→`VALUE=DATE`) leben als private Helfer in `IcsWriter`.
+- [x] `IcsEvent`-Eingabemodell (reine Kotlin: summary, start/end als Instant
+      + isAllDay + zoneId, recurrenceRule?, location, description,
+      reminderMinutes, status, availability, uid, calendarName) — entkoppelt
+      vom Provider-Modell
+- [x] `IcsWriter.writeCalendar(events, dtStamp)` → String: Header, pro Event
+      `VEVENT` nach Entscheidung #5, Zeitzonen-Regel #1, `VALARM`; JVM-Test
+      `IcsWriterTest` (all-day, getimt, wiederkehrend+TZID, unbekannte Zone,
+      Reminder, Escaping)
+- [x] UID-Ableitung `deriveIcsUid` (`uid ?: "<eventId>-<dtstartMillis>@calendula"`)
+      + Stabilitätstest
+
+**Provider → Domain (`data/calendar/IcsExportMapper.kt`):**
+- [x] Mapper Provider-Row → `IcsEvent` (`ColumnReader.toIcsEvent`) inkl.
+      DURATION→DTEND-Rekonstruktion (`parseRfc2445DurationMillis`),
+      `EventExportProjection`; Datasource-Methode `exportableEvents()` +
+      Repository `exportEvents()`; Test `IcsExportMapperTest`
+- [x] `insertEvent` schreibt `Events.UID_2445` (`UUID@calendula`) bei jedem
+      Create
+
+**Android-Export-Schicht:**
+- [x] `data/ics/IcsExporter`: `writeDocument(uri)` (SAF) + `stageShareFile`
+      (FileProvider-Cache) als UTF-8
+- [x] Einzel-Event-Share: Share-Action im Event-Detail → `IcsWriter` für ein
+      Event (one-off) → Cache-File über `FileProvider` → `ACTION_SEND`
+- [x] Ganz-Kalender-Backup: „Export as .ics file" in Settings → Calendars →
+      `ACTION_CREATE_DOCUMENT` → in den URI streamen; Ergebnis-Snackbar
+      (Plural „Exported N events")
+- [x] `FileProvider` + `file_paths.xml` im Manifest (Cache-Dir für Shares)
+- [x] Strings DE+EN: Share-Label/Chooser/Fehler, Backup-Sektion/Aktion/
+      Fehler + Plural, dateierter Default-Name
+
+**Abschluss:**
+- [ ] `./gradlew lint test assembleDebug` grün ← **nächster Schritt (Test)**
+- [x] CHANGELOG (`[Unreleased]`) ergänzt
+- [ ] On-Device-Review; **kein** Tag/Release vor Review und vor Merge von
+      Branch 2 (`feat/ics-import`)
+
+**Offene Detail-Calls (vor Review klären, nicht-blockierend):**
+- Kalender→Event-Mapping nutzt das per-`VEVENT`-Property `X-CALENDULA-CALENDAR`
+  (statt `X-WR-CALNAME`), damit eine kombinierte Datei mehrere Kalender trägt.
+- Backup = **eine** kombinierte `VCALENDAR`-Datei über alle lokalen Kalender.
+- EXDATE / `RECURRENCE-ID`-Ausnahmen werden beim Export ausgelassen
+  (`ORIGINAL_ID IS NULL`) — dokumentierter v1-Grenzfall, Import lässt sie auch aus.