fix(core-time): render all-day values in UTC, not the device zone

All-day items are date-only in iCalendar and are stored as UTC midnight of
their calendar date — the convention the tasks and calendar providers read
back (OpenTasks resolves them through DateTime.toAllDay(), which discards the
time and resolves the remaining date against UTC).

formatDate/formatDateTime/formatDateTimeCompact resolved every instant against
ZoneId.systemDefault(), so an all-day value rendered as the *previous* day for
anyone west of UTC: a task stored as 2026-07-20T00:00Z showed as 19 Jul in New
York. East of UTC it happened to land on the right day, which is why this went
unnoticed.

formatDate now takes an allDay flag (defaulting to false, so existing call
sites are unaffected) and all-day values resolve against UTC.

Callers must pass allDay at the call site — agendula and calendula both have
sites that need updating.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -6,14 +6,71 @@ All notable changes to floret-kit are documented here. The format follows
 floret-kit is consumed **from source** — each app embeds it as a git submodule
 and builds it through a Gradle composite build, pinning a specific commit. There
 are no binary releases; "version" tracks the shared `version` in the root build
-(currently `0.1.0`). Entries are grouped by module.
+(currently `0.2.1`). Entries are grouped by module.
 
 ## [Unreleased]
 
+## [0.2.1] - 2026-07-17
+
+### Added
+
+- **`components`** — `CollapsingScaffold` takes a `scrollable` flag (default
+  `true`, so every existing caller is unchanged). Set it to `false` when the
+  content brings its own scrolling container: the scaffold's own
+  `verticalScroll` composes each row up front and can't host a `LazyColumn`
+  (nesting a same-axis scrollable throws), which ruled out any surface with more
+  options than fit an eager pass. Opting out hands the content a plain column and
+  leaves the insets, IME padding and top-bar collapse as they were.
+  `FullScreenPicker` forwards the flag. Consumed by Calendula's ~600-entry
+  time-zone picker from 2.16.0.
+
+## [0.2.0] - 2026-07-17
+
+### Added
+
+- **`components`** — `CustomAmountEditor`, the amount + unit editor behind an
+  "Custom…" picker option (Calendula's custom snooze duration). Consumed by
+  Calendula since 2.15.0.
+- **`core-locale`** — the locale-aware formatting half of the module, so apps
+  stop hand-rolling `"$day. $month $year"` templates that bake one language's
+  field order (and its separators, and its ordinal dot) into every language they
+  ship:
+  - `localizedDateFormatter(locale, skeleton)` — a `DateTimeFormatter` for a
+    field skeleton ("LLLLy", "dMMMy", "EEEdMMM") laid out in the locale's own
+    order, via Android's best-pattern matching. Drop a field from the skeleton to
+    drop it from the output — the locale still supplies the arrangement.
+  - `currentLocale()` — the display locale, observed from `LocalConfiguration`.
+    Since `AppLanguage.apply()` changes the language inside a running process, a
+    locale captured once goes stale; this keys recomposition to the change.
+
+### Changed
+
+- **`core-locale`** now carries Compose (runtime + ui only) for `currentLocale()`.
+  The module previously declared itself Compose-free; applying a language
+  mid-process is its own API, so the hook that makes the UI notice belongs beside
+  it rather than in a Compose module depending back on this one. `core-crash` is
+  the precedent for a `core-*` module carrying Compose.
+
+### Known gaps
+
+- `core-time`'s `Instant.formatDate()` family now overlaps `core-locale`. Its
+  private `dateNoYearFormatter` strips the year by regex-ing the pattern string,
+  which `localizedDateFormatter(locale, "dMMM")` does properly; and its
+  formatters resolve `Locale.getDefault()` once at class-init ("locale changes
+  within a running process are rare enough to ignore"), which `AppLanguage.apply()`
+  makes untrue. Left alone for now — it is unused by Calendula but Agendula-visible.
+  Reconcile when Agendula next re-pins.
+
 ## [0.1.0] - 2026-07-07
 
 ### Added
 
+- **`core-locale`** (Android library) — per-app language plumbing (`AppLanguage`):
+  read the shipped languages from the app's `res/xml/locales_config.xml`, get/set
+  the applied language via `AppCompatDelegate`, render each language's autonym.
+  The app passes its own `locales_config` resource id; the system-default choice
+  is `null`. _(Shipped in 0.1.0 but omitted from this changelog at the time;
+  recorded here retroactively.)_
 - **`core-time`** (JVM library) — pure-Kotlin date/time helpers, no Android and
   no third-party dependencies:
   - `DayWindow.today()` — local-day boundaries for smart-list logic.

README.md

@@ -36,7 +36,7 @@ See [`docs/ARCHITECTURE.md`](docs/ARCHITECTURE.md) for the full consumption mode
 |--------|------|---------------|
 | `core-time` | JVM | Date/time helpers: `DayWindow` local-day windows, locale/zone-aware `Instant` formatting, `TimeBridge` (millis ↔ `Instant`). No Android, no deps. |
 | `core-reminders` | JVM | Reminder-lead plumbing: the lead-time unit model (`ReminderUnit`, `decomposeReminderMinutes`), the per-target override model (`ReminderOverride` + `reminderLeadFor` / `applyReminderOverride`) and the stored-format codec (`ReminderOverrideCodec`, separators configurable per app). Each app layers its own DataStore + string labels on top. |
-| `core-locale` | Android | Per-app language plumbing (`AppLanguage`): read the shipped languages from the app's `res/xml/locales_config.xml`, get/set the applied language via `AppCompatDelegate`, render each language's autonym. The app passes its own `locales_config`; appcompat only, no Compose. |
+| `core-locale` | Android | Everything "what language are we in". Per-app language plumbing (`AppLanguage`): read the shipped languages from the app's `res/xml/locales_config.xml`, get/set the applied language via `AppCompatDelegate`, render each language's autonym. Plus `currentLocale()` (observes the applied language from Compose) and `localizedDateFormatter(locale, skeleton)` (lays a field skeleton out in the locale's own order). The app passes its own `locales_config`. |
 | `core-crash` | Android | Privacy-respecting on-device crash capture + report dialog + issue-tracker hand-off. Parameterized per app via `CrashConfig`. |
 | `identity` | Android | The M3 Expressive theme factory `FloretExpressiveTheme(…)`, `rememberNavSlideSpec()`, the content transitions (`expandEnter`/`collapseExit`/`itemEnter`/…) and the `predictiveBack` peek. Each app supplies its own seed/palette. |
 | `components` | Android | Shared Compose vocabulary + recipes: `GroupedSurface`/`GroupedRow`, `InlineTextField`, `OptionCard`, `CollapsingScaffold`, `OptionPicker`, `ReorderableColumn`, `DebugRibbon`, `pastelize()`, `DialogControls`, `OnboardingScaffold`, `OptionalFormSection`, `AboutCard`, `LanguagePickerRow`. |

build.gradle.kts

@@ -10,5 +10,5 @@ plugins {
 
 subprojects {
     group = "de.jeanlucmakiola.floret"
-    version = "0.1.0"
+    version = "0.2.1"
 }

components/src/main/kotlin/de/jeanlucmakiola/floret/components/CollapsingScaffold.kt

@@ -46,6 +46,16 @@ import de.jeanlucmakiola.floret.identity.predictiveBack
  * unchanged: [actions] adds trailing app-bar items; [snackbarHost] hosts
  * transient messages; [predictiveBack] (off by default) wires the gesture
  * preview to [onBack] for full-screen surfaces that want it.
+ *
+ * Set [scrollable] to false when [content] brings its own scrolling container.
+ * The default column scrolls as a whole, which composes every row up front —
+ * fine for the short lists these surfaces normally hold, but a list long enough
+ * to need a [androidx.compose.foundation.lazy.LazyColumn] can neither nest one
+ * inside it (same-axis scrolling throws) nor afford the eager pass. Opting out
+ * hands [content] a plain, unscrolled column to fill instead; the insets, IME
+ * padding and bar behaviour are unchanged, so a lazy list simply takes over the
+ * scrolling and gets the top bar's collapse for free through the shared
+ * nested-scroll connection.
  */
 @OptIn(ExperimentalMaterial3Api::class)
 @Composable
@@ -55,6 +65,7 @@ fun CollapsingScaffold(
     modifier: Modifier = Modifier,
     largeTopBar: Boolean = true,
     predictiveBack: Boolean = false,
+    scrollable: Boolean = true,
     actions: @Composable RowScope.() -> Unit = {},
     snackbarHost: @Composable () -> Unit = {},
     content: @Composable ColumnScope.() -> Unit,
@@ -112,8 +123,19 @@ fun CollapsingScaffold(
                 .fillMaxSize()
                 .background(MaterialTheme.colorScheme.surface)
                 .imePadding()
-                .verticalScroll(rememberScrollState())
+                .then(
-                .padding(top = 8.dp, bottom = 24.dp),
+                    if (scrollable) {
+                        Modifier
+                            .verticalScroll(rememberScrollState())
+                            .padding(top = 8.dp, bottom = 24.dp)
+                    } else {
+                        // The content scrolls itself, so it also owns the edge
+                        // spacing — a lazy list wants it as contentPadding, or
+                        // the first and last rows clip instead of scrolling
+                        // under the bars.
+                        Modifier
+                    },
+                ),
             content = content,
         )
     }

components/src/main/kotlin/de/jeanlucmakiola/floret/components/DialogControls.kt

@@ -1,8 +1,11 @@
 package de.jeanlucmakiola.floret.components
 
+import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
+import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.Row
 import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.width
 import androidx.compose.foundation.shape.RoundedCornerShape
@@ -10,8 +13,12 @@ import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.filled.ArrowDropDown
 import androidx.compose.material3.DropdownMenu
 import androidx.compose.material3.DropdownMenuItem
+import androidx.compose.material3.FilledTonalButton
 import androidx.compose.material3.Icon
 import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.SegmentedButton
+import androidx.compose.material3.SegmentedButtonDefaults
+import androidx.compose.material3.SingleChoiceSegmentedButtonRow
 import androidx.compose.material3.Surface
 import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
@@ -57,6 +64,83 @@ fun DialogAmountField(
     }
 }
 
+/**
+ * The expanded "Custom" editor card shared by the family's presets-plus-custom
+ * pickers (reminder lead time, agenda range, snooze delay). A tonal surface with
+ * the connected-shape that meets the "Custom" row above it (tight top corners,
+ * full bottom), holding — top to bottom — an optional single-choice unit toggle,
+ * then a row of [DialogAmountField] + a live [preview] of what the amount
+ * resolves to + a tonal confirm.
+ *
+ * Purely presentational: the caller owns the amount/unit state and computes
+ * [preview] and [confirmEnabled] from its own domain, so this component carries
+ * no units, ranges, or strings of its own. Pass [unitLabels] empty for an
+ * amount-only editor (no toggle row). [onConfirm] fires on the confirm button.
+ */
+@Composable
+fun CustomAmountEditor(
+    amountText: String,
+    onAmountChange: (String) -> Unit,
+    preview: String,
+    setLabel: String,
+    confirmEnabled: Boolean,
+    onConfirm: () -> Unit,
+    modifier: Modifier = Modifier,
+    placeholder: String = "10",
+    unitLabels: List<String> = emptyList(),
+    selectedUnit: Int = 0,
+    onUnitChange: (Int) -> Unit = {},
+) {
+    Surface(
+        color = MaterialTheme.colorScheme.surfaceContainerHigh,
+        // A Position.Bottom shape: tight top corners meeting the row, full bottom.
+        shape = RoundedCornerShape(topStart = 6.dp, topEnd = 6.dp, bottomStart = 22.dp, bottomEnd = 22.dp),
+        modifier = modifier
+            .fillMaxWidth()
+            .padding(horizontal = 16.dp),
+    ) {
+        Column(
+            modifier = Modifier.padding(16.dp),
+            verticalArrangement = Arrangement.spacedBy(16.dp),
+        ) {
+            // Unit toggle first (when present) so it stays visible above the
+            // keyboard once the amount field below it is focused and scrolled in.
+            if (unitLabels.isNotEmpty()) {
+                SingleChoiceSegmentedButtonRow(modifier = Modifier.fillMaxWidth()) {
+                    unitLabels.forEachIndexed { index, label ->
+                        SegmentedButton(
+                            selected = index == selectedUnit,
+                            onClick = { onUnitChange(index) },
+                            shape = SegmentedButtonDefaults.itemShape(index, unitLabels.size),
+                            label = { Text(label) },
+                        )
+                    }
+                }
+            }
+            // Amount, a live preview of what it resolves to, and confirm — all on
+            // one row, sitting just above the keyboard.
+            Row(verticalAlignment = Alignment.CenterVertically) {
+                DialogAmountField(
+                    value = amountText,
+                    onValueChange = onAmountChange,
+                    placeholder = placeholder,
+                )
+                Spacer(Modifier.width(16.dp))
+                Text(
+                    text = preview,
+                    style = MaterialTheme.typography.bodyLarge,
+                    color = MaterialTheme.colorScheme.onSurfaceVariant,
+                    modifier = Modifier.weight(1f),
+                )
+                Spacer(Modifier.width(16.dp))
+                FilledTonalButton(onClick = onConfirm, enabled = confirmEnabled) {
+                    Text(setLabel)
+                }
+            }
+        }
+    }
+}
+
 /** Tonal dropdown trigger + menu shared by the custom reminder/recurrence steps and pickers. */
 @Composable
 fun DialogUnitDropdown(

components/src/main/kotlin/de/jeanlucmakiola/floret/components/OptionPicker.kt

@@ -20,12 +20,17 @@ import androidx.compose.ui.window.DialogWindowProvider
  * width without a tall header — it's a short selection list, so the large
  * collapsing header would only be empty space to scroll past. [content] places
  * the connected grouped rows; selecting one calls [onDismiss].
+ *
+ * [scrollable] forwards to [CollapsingScaffold]: leave it on unless [content]
+ * scrolls itself, which a picker only needs for an option list long enough to
+ * warrant a lazy container.
  */
 @Composable
 fun FullScreenPicker(
     title: String,
     onDismiss: () -> Unit,
     predictiveBack: Boolean = false,
+    scrollable: Boolean = true,
     actions: @Composable RowScope.() -> Unit = {},
     content: @Composable ColumnScope.() -> Unit,
 ) {
@@ -50,6 +55,7 @@ fun FullScreenPicker(
             onBack = onDismiss,
             largeTopBar = false,
             predictiveBack = predictiveBack,
+            scrollable = scrollable,
             actions = actions,
             content = content,
         )
@@ -96,8 +102,18 @@ fun <T> OptionPicker(
     }
 }
 
+/**
+ * The trailing check that marks the chosen row in a full-screen picker — the
+ * family's single selection affordance, so every picker (option, reminder,
+ * agenda range, timezone, calendar, recurrence) marks its selection the same
+ * way. Pair it with `GroupedRow(selected = true)`: the tonal highlight carries
+ * the state at a glance, the check confirms it unambiguously.
+ *
+ * Purely decorative — the row itself owns the semantics, so it takes no
+ * content description.
+ */
 @Composable
-private fun SelectedCheck() {
+fun SelectedCheck() {
     Icon(
         imageVector = Icons.Rounded.Check,
         contentDescription = null,

core-locale/build.gradle.kts

@@ -1,10 +1,20 @@
-// core-locale — per-app language plumbing shared across the family: read the
+// core-locale — everything "what language are we in", shared across the family:
-// shipped languages from res/xml/locales_config.xml, get/set the applied
+// read the shipped languages from res/xml/locales_config.xml, get/set the applied
-// language via AppCompatDelegate, and render each language's autonym. App-
+// language via AppCompatDelegate, render each language's autonym, observe the
-// agnostic — the consuming app passes its own locales_config resource id; no
+// current locale from Compose, and format dates in that locale's own field order.
-// Compose, no DataStore.
+// App-agnostic — the consuming app passes its own locales_config resource id; no
+// DataStore.
+//
+// Compose is here for currentLocale() alone: applying a language mid-process is
+// this module's own API, so the hook that makes the UI notice belongs beside it
+// rather than in a Compose module that would have to depend back on this one.
+// (core-crash is the precedent for a core-* module carrying Compose.) Deliberately
+// lean: runtime + ui only, no material3, no foundation.
 plugins {
+    // AGP 9.x provides built-in Kotlin compilation, so only the Compose plugin is
+    // applied alongside the Android library plugin.
     alias(libs.plugins.android.library)
+    alias(libs.plugins.kotlin.compose)
 }
 
 android {
@@ -20,6 +30,10 @@ android {
         targetCompatibility = JavaVersion.VERSION_17
     }
 
+    buildFeatures {
+        compose = true
+    }
+
     testOptions {
         unitTests {
             all { it.useJUnitPlatform() }
@@ -39,6 +53,16 @@ dependencies {
     // annotation (XmlRes) transitives.
     implementation(libs.androidx.appcompat)
 
+    // ConfigurationCompat for currentLocale(). Reachable transitively via
+    // appcompat, but declared because this module uses it directly.
+    implementation(libs.androidx.core.ktx)
+
+    // currentLocale() only needs the runtime (@Composable, remember) and ui
+    // (LocalConfiguration) — api() so consumers can use the returned Locale in
+    // their own composables without re-declaring Compose themselves.
+    api(platform(libs.androidx.compose.bom))
+    implementation(libs.androidx.ui)
+
     testImplementation(libs.junit.jupiter.api)
     testRuntimeOnly(libs.junit.jupiter.engine)
     testRuntimeOnly(libs.junit.platform.launcher)

core-locale/src/main/kotlin/de/jeanlucmakiola/floret/locale/DateFormatting.kt

@@ -0,0 +1,33 @@
+package de.jeanlucmakiola.floret.locale
+
+import java.time.format.DateTimeFormatter
+import java.util.Locale
+
+/**
+ * A [DateTimeFormatter] for [skeleton]'s fields laid out in [locale]'s own order,
+ * via Android's best-pattern matching.
+ *
+ * A skeleton lists the *fields* you want, not their arrangement: "dMMMy" asks for
+ * day, abbreviated month and year, and the locale decides that en-US renders
+ * "Jul 17, 2026" while de-DE renders "17. Juli 2026". Hand-rolled templates like
+ * `"$day. $month $year"` can't do this — they bake one locale's order (and its
+ * separators, and its ordinal dot) into every language the app ships.
+ *
+ * Drop a field to drop it from the output: "dMMM" is the same date without the
+ * year, still in the locale's order. Prefer that over post-processing a longer
+ * pattern, which cannot know which separators belonged to the removed field.
+ *
+ * Common skeletons: "LLLLy" (standalone month + year), "dMMMy" (day, month, year),
+ * "EEEdMMM" (weekday too). [locale] should come from [currentLocale] in Compose,
+ * so formatting follows an in-process language change.
+ *
+ * NB: this delegates to `android.text.format.DateFormat`, so it needs a real
+ * Android runtime — under unit tests with `isReturnDefaultValues` it yields no
+ * pattern. Test the skeleton *choice* in your own code; the layout itself is ICU's
+ * job and is not worth mocking.
+ */
+fun localizedDateFormatter(locale: Locale, skeleton: String): DateTimeFormatter =
+    DateTimeFormatter.ofPattern(
+        android.text.format.DateFormat.getBestDateTimePattern(locale, skeleton),
+        locale,
+    )

core-locale/src/main/kotlin/de/jeanlucmakiola/floret/locale/LocaleCompose.kt

@@ -0,0 +1,26 @@
+package de.jeanlucmakiola.floret.locale
+
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.remember
+import androidx.compose.ui.platform.LocalConfiguration
+import androidx.core.os.ConfigurationCompat
+import java.util.Locale
+
+/**
+ * The current display [Locale], read observably from [LocalConfiguration].
+ *
+ * Use this rather than [Locale.getDefault] anywhere a composable formats a date,
+ * a name or a number. [AppLanguage.apply] changes the language inside a running
+ * process, so a locale captured once — in a `val`, a top-level formatter, or a
+ * `remember {}` with no key — goes stale and keeps rendering the old language.
+ * Reading through [LocalConfiguration] keys recomposition to the configuration,
+ * so the UI follows the switch. (Android lint flags the alternative as
+ * NonObservableLocale.)
+ */
+@Composable
+fun currentLocale(): Locale {
+    val configuration = LocalConfiguration.current
+    return remember(configuration) {
+        ConfigurationCompat.getLocales(configuration).get(0) ?: Locale.getDefault()
+    }
+}

core-time/src/main/kotlin/de/jeanlucmakiola/floret/time/DateTimeFormat.kt

@@ -3,6 +3,7 @@ package de.jeanlucmakiola.floret.time
 import java.time.LocalDate
 import java.time.LocalTime
 import java.time.ZoneId
+import java.time.ZoneOffset
 import java.time.chrono.IsoChronology
 import java.time.format.DateTimeFormatter
 import java.time.format.DateTimeFormatterBuilder
@@ -22,15 +23,28 @@ private fun Instant.atSystemZone() =
     java.time.Instant.ofEpochMilli(toEpochMilliseconds())
         .atZone(ZoneId.systemDefault())
 
+/**
+ * The zone an instant should be read in to recover the date it denotes.
+ *
+ * All-day items are date-only in iCalendar and are stored as **UTC midnight** of
+ * their calendar date (the convention the tasks/calendar providers read back).
+ * Rendering one in the device zone shows the previous day for anyone west of
+ * UTC — so all-day values resolve against UTC, timed values against the device.
+ */
+private fun Instant.atDisplayZone(allDay: Boolean) =
+    if (allDay) java.time.Instant.ofEpochMilli(toEpochMilliseconds()).atZone(ZoneOffset.UTC)
+    else atSystemZone()
+
 /** Medium localized date, e.g. "18 Jun 2026". */
-fun Instant.formatDate(): String = atSystemZone().format(dateFormatter)
+fun Instant.formatDate(allDay: Boolean = false): String =
+    atDisplayZone(allDay).format(dateFormatter)
 
 /** Short localized time, e.g. "14:30". */
 fun Instant.formatTime(): String = atSystemZone().format(timeFormatter)
 
 /** Date alone for all-day items, otherwise date + time. */
 fun Instant.formatDateTime(allDay: Boolean): String =
-    if (allDay) formatDate() else "${formatDate()} · ${formatTime()}"
+    if (allDay) formatDate(allDay = true) else "${formatDate()} · ${formatTime()}"
 
 // The locale's medium date pattern with the year token stripped, e.g. "20 Jun" /
 // "Jun 20" — derived from the pattern so day/month order still follows the locale.
@@ -49,10 +63,10 @@ private val dateNoYearFormatter: DateTimeFormatter = run {
  * items, or a midnight time). Examples: "20 Jun", "20 Jun · 14:30", "20 Jun 2027".
  */
 fun Instant.formatDateTimeCompact(allDay: Boolean): String {
-    val zdt = atSystemZone()
+    val zdt = atDisplayZone(allDay)
     val datePart =
         if (zdt.year == LocalDate.now(ZoneId.systemDefault()).year) zdt.format(dateNoYearFormatter)
-        else formatDate()
+        else formatDate(allDay)
     return if (allDay || zdt.toLocalTime() == LocalTime.MIDNIGHT) datePart
     else "$datePart · ${formatTime()}"
 }

core-time/src/test/kotlin/de/jeanlucmakiola/floret/time/DateTimeFormatTest.kt

@@ -0,0 +1,76 @@
+package de.jeanlucmakiola.floret.time
+
+import com.google.common.truth.Truth.assertThat
+import org.junit.jupiter.api.AfterEach
+import org.junit.jupiter.api.Test
+import java.time.LocalDate
+import java.time.ZoneOffset
+import java.time.format.DateTimeFormatter
+import java.time.format.FormatStyle
+import java.util.TimeZone
+import kotlin.time.Instant
+
+class DateTimeFormatTest {
+
+    private val originalZone: TimeZone = TimeZone.getDefault()
+
+    @AfterEach
+    fun restoreZone() = TimeZone.setDefault(originalZone)
+
+    private val july20: LocalDate = LocalDate.of(2026, 7, 20)
+
+    /** The all-day storage form: UTC midnight of the calendar date. */
+    private val allDayJuly20: Instant = Instant.fromEpochMilliseconds(
+        july20.atStartOfDay(ZoneOffset.UTC).toInstant().toEpochMilli(),
+    )
+
+    /** What the locale renders for a given date — so the test doesn't pin a format. */
+    private fun medium(date: LocalDate): String =
+        date.format(DateTimeFormatter.ofLocalizedDate(FormatStyle.MEDIUM))
+
+    @Test
+    fun `an all-day date reads the same whatever the device zone`() {
+        TimeZone.setDefault(TimeZone.getTimeZone("America/New_York")) // UTC-4 in July
+        assertThat(allDayJuly20.formatDate(allDay = true)).isEqualTo(medium(july20))
+
+        TimeZone.setDefault(TimeZone.getTimeZone("Europe/Berlin")) // UTC+2 in July
+        assertThat(allDayJuly20.formatDate(allDay = true)).isEqualTo(medium(july20))
+
+        TimeZone.setDefault(TimeZone.getTimeZone("Pacific/Auckland")) // UTC+12
+        assertThat(allDayJuly20.formatDate(allDay = true)).isEqualTo(medium(july20))
+    }
+
+    @Test
+    fun `reading an all-day value as timed slips a day west of UTC`() {
+        // Guards the regression directly: this is what the all-day flag prevents.
+        TimeZone.setDefault(TimeZone.getTimeZone("America/New_York"))
+        assertThat(allDayJuly20.formatDate()).isEqualTo(medium(july20.minusDays(1)))
+        assertThat(allDayJuly20.formatDate(allDay = true)).isEqualTo(medium(july20))
+    }
+
+    @Test
+    fun `timed values still render in the device zone`() {
+        // 2026-07-20T23:30Z — the next day already, in Auckland.
+        val lateUtc = Instant.fromEpochMilliseconds(
+            july20.atTime(23, 30).atZone(ZoneOffset.UTC).toInstant().toEpochMilli(),
+        )
+        TimeZone.setDefault(TimeZone.getTimeZone("Pacific/Auckland"))
+        assertThat(lateUtc.formatDate()).isEqualTo(medium(july20.plusDays(1)))
+    }
+
+    @Test
+    fun `formatDateTime drops the time for all-day and keeps it otherwise`() {
+        TimeZone.setDefault(TimeZone.getTimeZone("Europe/Berlin"))
+        assertThat(allDayJuly20.formatDateTime(allDay = true)).isEqualTo(medium(july20))
+        assertThat(allDayJuly20.formatDateTime(allDay = false)).contains("·")
+    }
+
+    @Test
+    fun `compact form is zone-correct for all-day too`() {
+        TimeZone.setDefault(TimeZone.getTimeZone("America/New_York"))
+        val compact = allDayJuly20.formatDateTimeCompact(allDay = true)
+        // No time part, and the day must not have slipped to the 19th.
+        assertThat(compact).doesNotContain("·")
+        assertThat(compact).contains("20")
+    }
+}

docs/ROADMAP.md

@@ -13,6 +13,7 @@ divergent or app-specific code where it lives.
 |---|---|---|
 | `core-time` | JVM | Agendula (and Calendula's pending branch) |
 | `core-reminders` | JVM | Agendula (and Calendula's pending branch) |
+| `core-locale` | Android | Agendula, Calendula |
 | `core-crash` | Android | Agendula |
 | `identity` | Android | Agendula |
 | `components` | Android | Agendula |