app/src/main/AndroidManifest.xml

@@ -112,6 +112,17 @@
                 <data android:mimeType="text/calendar" />
             </intent-filter>
 
+            <!-- Let another app or widget (e.g. the Todo Agenda widget) launch us
+                 to create a new event, the way the AOSP calendar accepts it:
+                 ACTION_INSERT on the events dir mime type, carrying the new
+                 event's fields as CalendarContract extras
+                 (MainActivity.insertFormOrNull, issue #30). -->
+            <intent-filter>
+                <action android:name="android.intent.action.INSERT" />
+                <category android:name="android.intent.category.DEFAULT" />
+                <data android:mimeType="vnd.android.cursor.dir/event" />
+            </intent-filter>
+
             <!-- Launcher long-press shortcuts (e.g. "New event"). -->
             <meta-data
                 android:name="android.app.shortcuts"

app/src/main/java/de/jeanlucmakiola/calendula/MainActivity.kt

@@ -4,6 +4,7 @@ import android.content.Context
 import android.content.Intent
 import android.net.Uri
 import android.os.Bundle
+import android.provider.CalendarContract
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
 import androidx.appcompat.app.AppCompatActivity
@@ -25,6 +26,8 @@ import dagger.hilt.android.AndroidEntryPoint
 import de.jeanlucmakiola.calendula.data.crash.CrashReporter
 import de.jeanlucmakiola.calendula.data.prefs.ThemeMode
 import de.jeanlucmakiola.calendula.data.prefs.is24Hour
+import de.jeanlucmakiola.calendula.domain.EventForm
+import de.jeanlucmakiola.calendula.domain.buildInsertEventForm
 import de.jeanlucmakiola.calendula.ui.RootScreen
 import de.jeanlucmakiola.calendula.ui.common.LocalShowHourLines
 import de.jeanlucmakiola.calendula.ui.common.LocalUse24HourFormat
@@ -42,6 +45,7 @@ import de.jeanlucmakiola.calendula.ui.theme.resolveFontFamily
 import kotlinx.datetime.LocalDate
 import kotlinx.datetime.TimeZone
 import kotlinx.datetime.toLocalDateTime
+import kotlin.time.Clock
 import kotlin.time.Instant
 
 @AndroidEntryPoint
@@ -60,6 +64,11 @@ class MainActivity : AppCompatActivity() {
     // by CalendarHost's import flow.
     private var requestedImportUri by mutableStateOf<Uri?>(null)
 
+    // A prefilled new-event form from an external ACTION_INSERT launch (another
+    // app/widget asking us to create an event, issue #30). Consumed once by
+    // CalendarHost, which opens it in the create form for review.
+    private var requestedInsertForm by mutableStateOf<EventForm?>(null)
+
     // A captured crash report awaiting the user's decision, surfaced as a dialog
     // over the calendar on the next launch (the single-crash path). A startup
     // crash-loop is handled out of band, before setContent — see below.
@@ -84,6 +93,7 @@ class MainActivity : AppCompatActivity() {
         requestedDetailKey = intent.detailKeyOrNull()
         requestedNav = intent.navRequestOrNull()
         requestedImportUri = intent.importUriOrNull()
+        requestedInsertForm = intent.insertFormOrNull()
         if (CrashReporter.shouldPrompt(this)) pendingCrashReport = CrashReporter.pendingReport(this)
         setContent {
             // One activity-scoped SettingsViewModel drives both the theme here
@@ -132,6 +142,8 @@ class MainActivity : AppCompatActivity() {
                             onWidgetNavConsumed = { requestedNav = null },
                             requestedImportUri = requestedImportUri,
                             onImportConsumed = { requestedImportUri = null },
+                            requestedInsertForm = requestedInsertForm,
+                            onInsertConsumed = { requestedInsertForm = null },
                         )
                     }
                     // A persistent corner marker so a debug build is never
@@ -169,6 +181,7 @@ class MainActivity : AppCompatActivity() {
         intent.detailKeyOrNull()?.let { requestedDetailKey = it }
         intent.navRequestOrNull()?.let { requestedNav = it }
         intent.importUriOrNull()?.let { requestedImportUri = it }
+        intent.insertFormOrNull()?.let { requestedInsertForm = it }
     }
 
     /**
@@ -188,6 +201,31 @@ class MainActivity : AppCompatActivity() {
         return uri.takeIf { it.scheme == "content" || it.scheme == "file" }
     }
 
+    /**
+     * A prefilled new-event form from an external `ACTION_INSERT` launch — another
+     * app or widget (e.g. Todo Agenda) asking us to create an event (issue #30).
+     * The new event's fields ride as CalendarContract extras; anything omitted
+     * falls back to the in-app "new event" defaults in [buildInsertEventForm].
+     */
+    private fun Intent.insertFormOrNull(): EventForm? {
+        if (action != Intent.ACTION_INSERT) return null
+        return buildInsertEventForm(
+            beginMillis = longExtraOrNull(CalendarContract.EXTRA_EVENT_BEGIN_TIME),
+            endMillis = longExtraOrNull(CalendarContract.EXTRA_EVENT_END_TIME),
+            isAllDay = getBooleanExtra(CalendarContract.EXTRA_EVENT_ALL_DAY, false),
+            title = getStringExtra(CalendarContract.Events.TITLE),
+            description = getStringExtra(CalendarContract.Events.DESCRIPTION),
+            location = getStringExtra(CalendarContract.Events.EVENT_LOCATION),
+            rrule = getStringExtra(CalendarContract.Events.RRULE),
+            zone = TimeZone.currentSystemDefault(),
+            now = Clock.System.now(),
+        )
+    }
+
+    /** A Long extra's value, or null when the extra is absent. */
+    private fun Intent.longExtraOrNull(key: String): Long? =
+        if (hasExtra(key)) getLongExtra(key, 0L) else null
+
     /**
      * The date a launcher/clock date tap points at, parsed from the AOSP calendar
      * "view time" intent: ACTION_VIEW on `content://com.android.calendar/time/

app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/EventDetailMapper.kt

@@ -23,26 +23,29 @@ internal fun ColumnReader.toEventDetailCore(
     attendees: List<Attendee>,
     reminders: List<Reminder>,
 ): EventDetail? {
-    val begin = getLong(EventDetailProjection.IDX_DTSTART)
-
-    if (begin < 0L) {
-        Log.w(TAG, "Dropping event with negative dtstart=$begin")
+    // DTSTART is epoch millis in UTC, so a series anchored before 1970 (common
+    // for yearly birthdays/anniversaries synced over CalDAV) is legitimately
+    // negative — only an *absent* DTSTART marks a malformed row worth dropping.
+    // Dropping negatives made every occurrence of such a series un-openable
+    // (the detail loads the ancient series-master DTSTART), see issue #34.
+    if (isNull(EventDetailProjection.IDX_DTSTART)) {
+        Log.w(TAG, "Dropping event with missing dtstart")
         return null
     }
+    val begin = getLong(EventDetailProjection.IDX_DTSTART)
 
     // Recurring events store DURATION instead of DTEND, so the series row's
     // DTEND is null. Keep the event (end == begin); callers that opened a
     // specific occurrence supply the real per-occurrence times from
-    // CalendarContract.Instances. Only a present-but-backwards DTEND is malformed.
+    // CalendarContract.Instances. A present-but-backwards DTEND is malformed,
+    // but dropping the row would make the event un-openable — the same trap as
+    // the pre-1970 DTSTART bug above (issue #34): it would surface as the
+    // generic error screen with no way to open the event and fix it. Clamp to a
+    // zero-length event instead (matching SearchMapper's coerceAtLeast).
     val end = if (isNull(EventDetailProjection.IDX_DTEND)) {
         begin
     } else {
-        val rawEnd = getLong(EventDetailProjection.IDX_DTEND)
-        if (rawEnd < begin) {
-            Log.w(TAG, "Dropping event with dtend=$rawEnd < dtstart=$begin")
-            return null
-        }
-        rawEnd
+        getLong(EventDetailProjection.IDX_DTEND).coerceAtLeast(begin)
     }
 
     // Kept raw (no untitled fallback): the detail screen substitutes its own

app/src/main/java/de/jeanlucmakiola/calendula/data/calendar/SearchMapper.kt

@@ -10,8 +10,11 @@ import de.jeanlucmakiola.calendula.domain.ics.parseRfc2445DurationMillis
  * of DTEND — reconstruct the end the same way the `.ics` export does.
  */
 internal fun ColumnReader.toSearchResult(): EventInstance? {
+    // A pre-1970 series anchor is a legitimately negative epoch-millis DTSTART
+    // (see EventDetailMapper / issue #34); drop only a genuinely absent one, so
+    // long-running birthdays/anniversaries still surface in search.
+    if (isNull(SearchProjection.IDX_DTSTART)) return null
     val dtStart = getLong(SearchProjection.IDX_DTSTART)
-    if (dtStart < 0L) return null
     val end = when {
         !isNull(SearchProjection.IDX_DTEND) -> getLong(SearchProjection.IDX_DTEND)
         else -> dtStart + parseRfc2445DurationMillis(getString(SearchProjection.IDX_DURATION))

app/src/main/java/de/jeanlucmakiola/calendula/domain/InsertEventForm.kt

@@ -0,0 +1,76 @@
+package de.jeanlucmakiola.calendula.domain
+
+import kotlinx.datetime.LocalDate
+import kotlinx.datetime.LocalDateTime
+import kotlinx.datetime.LocalTime
+import kotlinx.datetime.TimeZone
+import kotlinx.datetime.toInstant
+import kotlinx.datetime.toLocalDateTime
+import kotlin.time.Duration.Companion.hours
+import kotlin.time.Instant
+
+/**
+ * Build a prefilled [EventForm] from an `ACTION_INSERT` intent's extras (issue
+ * #30). External apps and widgets (e.g. the Todo Agenda widget) launch the
+ * calendar this way to create a new event, passing the fields as
+ * [android.provider.CalendarContract] extras. Any field the intent omits falls
+ * back to the same defaults the in-app "new event" uses — a timed start at the
+ * next full hour and a one-hour duration. [EventForm.calendarId] is left null so
+ * it resolves to the last-used / first-writable calendar, exactly like the
+ * `.ics` single-event and plain new-event paths.
+ *
+ * Pure (no Android types) so it is unit-testable; the intent parsing that reads
+ * the extras lives in `MainActivity.insertFormOrNull`.
+ */
+fun buildInsertEventForm(
+    beginMillis: Long?,
+    endMillis: Long?,
+    isAllDay: Boolean,
+    title: String?,
+    description: String?,
+    location: String?,
+    rrule: String?,
+    zone: TimeZone,
+    now: Instant,
+): EventForm {
+    val (start, end) = if (isAllDay) {
+        // All-day provider times are UTC midnights with an exclusive end; show
+        // the last covered day and keep placeholder wall-clock times in case the
+        // user switches the event to timed (mirrors EventDetail.toEditForm).
+        val startDate = beginMillis
+            ?.let { Instant.fromEpochMilliseconds(it).toLocalDateTime(TimeZone.UTC).date }
+            ?: now.toLocalDateTime(zone).date
+        val endDate = endMillis
+            ?.let { Instant.fromEpochMilliseconds(it).toLocalDateTime(TimeZone.UTC).date }
+            ?.let { exclusive -> maxOf(startDate, LocalDate.fromEpochDays(exclusive.toEpochDays() - 1)) }
+            ?: startDate
+        LocalDateTime(startDate, LocalTime(9, 0)) to LocalDateTime(endDate, LocalTime(10, 0))
+    } else {
+        val startTime = beginMillis
+            ?.let { Instant.fromEpochMilliseconds(it).toLocalDateTime(zone) }
+            ?: nextFullHour(now, zone)
+        val endTime = endMillis
+            ?.let { Instant.fromEpochMilliseconds(it).toLocalDateTime(zone) }
+            ?.takeIf { it >= startTime }
+            ?: (startTime.toInstant(zone) + 1.hours).toLocalDateTime(zone)
+        startTime to endTime
+    }
+    return EventForm(
+        calendarId = null,
+        title = title.orEmpty(),
+        isAllDay = isAllDay,
+        start = start,
+        end = end,
+        location = location.orEmpty(),
+        description = description.orEmpty(),
+        // Bare RRULE value (Events.RRULE convention); tolerate a leading "RRULE:"
+        // some callers include.
+        rrule = rrule?.removePrefix("RRULE:")?.takeIf { it.isNotBlank() },
+    )
+}
+
+private fun nextFullHour(now: Instant, zone: TimeZone): LocalDateTime {
+    val hourMillis = 3_600_000L
+    val rounded = (now.toEpochMilliseconds() / hourMillis + 1) * hourMillis
+    return Instant.fromEpochMilliseconds(rounded).toLocalDateTime(zone)
+}

app/src/main/java/de/jeanlucmakiola/calendula/ui/CalendarHost.kt

@@ -72,6 +72,8 @@ fun CalendarHost(
     onWidgetNavConsumed: () -> Unit = {},
     requestedImportUri: android.net.Uri? = null,
     onImportConsumed: () -> Unit = {},
+    requestedInsertForm: EventForm? = null,
+    onInsertConsumed: () -> Unit = {},
     viewModel: CalendarHostViewModel = hiltViewModel(),
 ) {
     // Wait for the persisted default view before seeding the stack, so the app
@@ -176,6 +178,16 @@ fun CalendarHost(
             onImportConsumed()
         }
     }
+    // An external ACTION_INSERT launch (another app/widget creating an event,
+    // issue #30) arrives already prefilled — open it in the same create form the
+    // single-event .ics path uses. [importForm] is the topmost overlay, so it
+    // reveals on top of whatever was open without extra dismissal.
+    LaunchedEffect(requestedInsertForm) {
+        if (requestedInsertForm != null) {
+            importForm = requestedInsertForm
+            onInsertConsumed()
+        }
+    }
 
     // Close every overlay that can sit over the calendar, so an externally
     // requested destination (a widget/shortcut/QS-tile launch) is revealed on

app/src/main/java/de/jeanlucmakiola/calendula/ui/RootScreen.kt

@@ -35,6 +35,8 @@ fun RootScreen(
     onWidgetNavConsumed: () -> Unit = {},
     requestedImportUri: android.net.Uri? = null,
     onImportConsumed: () -> Unit = {},
+    requestedInsertForm: de.jeanlucmakiola.calendula.domain.EventForm? = null,
+    onInsertConsumed: () -> Unit = {},
 ) {
     val context = LocalContext.current
     var hasPermission by remember {
@@ -84,6 +86,8 @@ fun RootScreen(
                         onWidgetNavConsumed = onWidgetNavConsumed,
                         requestedImportUri = requestedImportUri,
                         onImportConsumed = onImportConsumed,
+                        requestedInsertForm = requestedInsertForm,
+                        onInsertConsumed = onInsertConsumed,
                     )
                     false -> ReminderOnboardingScreen(
                         onFinished = reminderOnboarding::finish,

app/src/main/java/de/jeanlucmakiola/calendula/ui/common/TimePickerAlert.kt

@@ -4,14 +4,9 @@ import androidx.compose.material3.AlertDialog
 import androidx.compose.material3.ExperimentalMaterial3Api
 import androidx.compose.material3.Text
 import androidx.compose.material3.TextButton
-import android.content.Context
-import android.content.res.Resources
-import android.provider.Settings
-import android.text.format.DateFormat
 import androidx.compose.material3.TimePicker
 import androidx.compose.material3.rememberTimePickerState
 import androidx.compose.runtime.Composable
-import androidx.compose.ui.platform.LocalContext
 import androidx.compose.ui.res.stringResource
 import de.jeanlucmakiola.calendula.R
 import kotlinx.datetime.LocalTime
@@ -27,10 +22,13 @@ fun TimePickerAlert(
     onConfirm: (LocalTime) -> Unit,
     onDismiss: () -> Unit,
 ) {
+    // Honour the app's own time-format preference (the value every time label
+    // reads), not the system/locale clock — otherwise an explicit 24h setting
+    // still showed an AM/PM dial (issue #27).
     val state = rememberTimePickerState(
         initialHour = initial.hour,
         initialMinute = initial.minute,
-        is24Hour = deviceUses24HourClock(LocalContext.current),
+        is24Hour = LocalUse24HourFormat.current,
     )
     AlertDialog(
         onDismissRequest = onDismiss,
@@ -45,24 +43,3 @@ fun TimePickerAlert(
         text = { TimePicker(state = state) },
     )
 }
-
-/**
- * Whether the clock should read 24-hour, matching the rest of the device.
- *
- * [DateFormat.is24HourFormat] resolves a "locale default" system setting against
- * the *app's* context locale — and this app applies a per-app language
- * (AppCompatDelegate), so an English UI on a German-region phone would wrongly
- * read 12-hour while the system clock shows 24-hour. So we honour an explicit
- * system 12/24 override, and otherwise fall back to the **device** locale
- * (Resources.getSystem), not the app's.
- */
-private fun deviceUses24HourClock(context: Context): Boolean =
-    when (Settings.System.getString(context.contentResolver, Settings.System.TIME_12_24)) {
-        "24" -> true
-        "12" -> false
-        // 'a' is the AM/PM marker; a best-fit pattern without it is 24-hour.
-        else -> {
-            val deviceLocale = Resources.getSystem().configuration.locales[0]
-            !DateFormat.getBestDateTimePattern(deviceLocale, "jm").contains('a')
-        }
-    }

app/src/test/java/de/jeanlucmakiola/calendula/data/calendar/EventDetailMapperTest.kt

@@ -22,8 +22,8 @@ class EventDetailMapperTest {
         eventColor: Any? = null,
         eventColorKey: String? = null,
         calendarColor: Int = 0xFFAABBCC.toInt(),
-        dtstart: Long = 1_000_000_000L,
-        dtend: Long = 1_000_003_600L,
+        dtstart: Long? = 1_000_000_000L,
+        dtend: Long? = 1_000_003_600L,
         allDay: Int = 0,
         location: String? = "Berlin",
         calendarId: Long = 7L,
@@ -120,9 +120,31 @@ class EventDetailMapperTest {
     }
 
     @Test
-    fun `dtend before dtstart drops detail`() {
+    fun `dtend before dtstart is clamped to a zero-length event, not dropped`() {
+        // A backwards DTEND is malformed, but dropping it would make the event
+        // un-openable (the "Something went wrong" trap, same as issue #34); keep
+        // it as a zero-length event so the user can still open and fix it.
         val detail = detailReader(dtstart = 2000L, dtend = 1000L).toDetail()
-        assertThat(detail).isNull()
+        assertThat(detail).isNotNull()
+        assertThat(detail!!.instance.start.toEpochMilliseconds()).isEqualTo(2000L)
+        assertThat(detail.instance.end.toEpochMilliseconds()).isEqualTo(2000L)
+    }
+
+    @Test
+    fun `pre-1970 negative dtstart is kept, not dropped (issue #34)`() {
+        // A yearly birthday/anniversary anchored before the epoch has a
+        // legitimately negative UTC epoch-millis DTSTART; recurring rows carry
+        // no DTEND (they use DURATION), so it stays end == begin.
+        val begin = -157_766_400_000L // 1965-01-01T00:00:00Z
+        val detail = detailReader(dtstart = begin, dtend = null).toDetail()
+        assertThat(detail).isNotNull()
+        assertThat(detail!!.instance.start.toEpochMilliseconds()).isEqualTo(begin)
+        assertThat(detail.instance.end.toEpochMilliseconds()).isEqualTo(begin)
+    }
+
+    @Test
+    fun `absent dtstart drops detail`() {
+        assertThat(detailReader(dtstart = null).toDetail()).isNull()
     }
 
     @Test

app/src/test/java/de/jeanlucmakiola/calendula/data/calendar/SearchMapperTest.kt

@@ -0,0 +1,45 @@
+package de.jeanlucmakiola.calendula.data.calendar
+
+import com.google.common.truth.Truth.assertThat
+import org.junit.jupiter.api.Test
+
+class SearchMapperTest {
+
+    private fun searchReader(
+        id: Long = 1L,
+        calendarId: Long = 7L,
+        title: String? = "Birthday",
+        dtstart: Long? = 1_000_000_000L,
+        dtend: Long? = null,
+        duration: String? = "P1D",
+        allDay: Int = 1,
+        eventColor: Any? = null,
+        calendarColor: Int = 0xFFAABBCC.toInt(),
+        location: String? = null,
+    ): MapColumnReader = MapColumnReader(
+        SearchProjection.IDX_ID to id,
+        SearchProjection.IDX_CALENDAR_ID to calendarId,
+        SearchProjection.IDX_TITLE to title,
+        SearchProjection.IDX_DTSTART to dtstart,
+        SearchProjection.IDX_DTEND to dtend,
+        SearchProjection.IDX_DURATION to duration,
+        SearchProjection.IDX_ALL_DAY to allDay,
+        SearchProjection.IDX_EVENT_COLOR to eventColor,
+        SearchProjection.IDX_CALENDAR_COLOR to calendarColor,
+        SearchProjection.IDX_LOCATION to location,
+    )
+
+    @Test
+    fun `pre-1970 negative dtstart still surfaces in search (issue #34)`() {
+        val begin = -157_766_400_000L // 1965-01-01T00:00:00Z
+        val result = searchReader(dtstart = begin, dtend = null, duration = "P1D").toSearchResult()
+        assertThat(result).isNotNull()
+        assertThat(result!!.start.toEpochMilliseconds()).isEqualTo(begin)
+        assertThat(result.title).isEqualTo("Birthday")
+    }
+
+    @Test
+    fun `absent dtstart drops the search hit`() {
+        assertThat(searchReader(dtstart = null).toSearchResult()).isNull()
+    }
+}

app/src/test/java/de/jeanlucmakiola/calendula/domain/InsertEventFormTest.kt

@@ -0,0 +1,115 @@
+package de.jeanlucmakiola.calendula.domain
+
+import com.google.common.truth.Truth.assertThat
+import kotlinx.datetime.LocalDate
+import kotlinx.datetime.LocalDateTime
+import kotlinx.datetime.LocalTime
+import kotlinx.datetime.TimeZone
+import kotlinx.datetime.atStartOfDayIn
+import kotlinx.datetime.toInstant
+import org.junit.jupiter.api.Test
+import kotlin.time.Instant
+
+class InsertEventFormTest {
+
+    private val zone = TimeZone.UTC
+
+    // A fixed "now" at 2026-01-15T10:20:00Z; the next full hour is 11:00.
+    private val now = LocalDateTime(2026, 1, 15, 10, 20).toInstant(zone)
+
+    private fun build(
+        beginMillis: Long? = null,
+        endMillis: Long? = null,
+        isAllDay: Boolean = false,
+        title: String? = null,
+        description: String? = null,
+        location: String? = null,
+        rrule: String? = null,
+    ): EventForm = buildInsertEventForm(
+        beginMillis = beginMillis,
+        endMillis = endMillis,
+        isAllDay = isAllDay,
+        title = title,
+        description = description,
+        location = location,
+        rrule = rrule,
+        zone = zone,
+        now = now,
+    )
+
+    private fun millis(dateTime: LocalDateTime): Long = dateTime.toInstant(zone).toEpochMilliseconds()
+
+    @Test
+    fun `timed event maps all provided fields`() {
+        val start = LocalDateTime(2026, 3, 1, 14, 30)
+        val end = LocalDateTime(2026, 3, 1, 15, 45)
+        val form = build(
+            beginMillis = millis(start),
+            endMillis = millis(end),
+            title = "Standup",
+            description = "Daily",
+            location = "Room 1",
+            rrule = "FREQ=DAILY",
+        )
+        assertThat(form.calendarId).isNull() // resolves to last-used / first-writable
+        assertThat(form.isAllDay).isFalse()
+        assertThat(form.start).isEqualTo(start)
+        assertThat(form.end).isEqualTo(end)
+        assertThat(form.title).isEqualTo("Standup")
+        assertThat(form.description).isEqualTo("Daily")
+        assertThat(form.location).isEqualTo("Room 1")
+        assertThat(form.rrule).isEqualTo("FREQ=DAILY")
+    }
+
+    @Test
+    fun `missing times default to the next full hour and a one-hour duration`() {
+        val form = build(title = "Quick note")
+        assertThat(form.start).isEqualTo(LocalDateTime(2026, 1, 15, 11, 0))
+        assertThat(form.end).isEqualTo(LocalDateTime(2026, 1, 15, 12, 0))
+        assertThat(form.title).isEqualTo("Quick note")
+    }
+
+    @Test
+    fun `timed event with only a start gets a one-hour end`() {
+        val start = LocalDateTime(2026, 5, 2, 9, 0)
+        val form = build(beginMillis = millis(start))
+        assertThat(form.start).isEqualTo(start)
+        assertThat(form.end).isEqualTo(LocalDateTime(2026, 5, 2, 10, 0))
+    }
+
+    @Test
+    fun `an end before the start is ignored and falls back to plus one hour`() {
+        val start = LocalDateTime(2026, 5, 2, 9, 0)
+        val badEnd = LocalDateTime(2026, 5, 2, 8, 0)
+        val form = build(beginMillis = millis(start), endMillis = millis(badEnd))
+        assertThat(form.end).isEqualTo(LocalDateTime(2026, 5, 2, 10, 0))
+    }
+
+    @Test
+    fun `all-day event uses UTC dates and decrements the exclusive end`() {
+        val startMillis = LocalDate(2026, 1, 1).atStartOfDayIn(TimeZone.UTC).toEpochMilliseconds()
+        val endExclusive = LocalDate(2026, 1, 3).atStartOfDayIn(TimeZone.UTC).toEpochMilliseconds()
+        val form = build(beginMillis = startMillis, endMillis = endExclusive, isAllDay = true, title = "Trip")
+        assertThat(form.isAllDay).isTrue()
+        assertThat(form.start.date).isEqualTo(LocalDate(2026, 1, 1))
+        assertThat(form.end.date).isEqualTo(LocalDate(2026, 1, 2))
+        // Placeholder wall-clock times survive a switch back to a timed event.
+        assertThat(form.start.time).isEqualTo(LocalTime(9, 0))
+        assertThat(form.end.time).isEqualTo(LocalTime(10, 0))
+    }
+
+    @Test
+    fun `all-day event without an end is a single day`() {
+        val startMillis = LocalDate(2026, 6, 10).atStartOfDayIn(TimeZone.UTC).toEpochMilliseconds()
+        val form = build(beginMillis = startMillis, isAllDay = true)
+        assertThat(form.start.date).isEqualTo(LocalDate(2026, 6, 10))
+        assertThat(form.end.date).isEqualTo(LocalDate(2026, 6, 10))
+    }
+
+    @Test
+    fun `a leading RRULE prefix is stripped and a blank rule becomes null`() {
+        assertThat(build(rrule = "RRULE:FREQ=WEEKLY").rrule).isEqualTo("FREQ=WEEKLY")
+        assertThat(build(rrule = "   ").rrule).isNull()
+        assertThat(build(rrule = null).rrule).isNull()
+    }
+}