fix(edit): correct the recurrence picker's read-out and end handling

Review follow-ups to the picker redesign, plus the bugs it exposed.

The live read-out now renders customResult itself — the exact string OK
would save — instead of rebuilding a parallel rule with `interval ?: 1`
and `end ?: Never` fallbacks, which let it confidently describe a rule
that differed from the selected controls whenever a field was invalid.

Shrink the invalid space behind it: a blank amount field reads as its
visible placeholder (1 / 10) rather than as an error, and backing out of
the date picker falls back to "never" instead of stranding a dateless
"on a date". Only an out-of-range 0 remains invalid, and that now says so
rather than greying out OK with no cause.

UNTIL displayed the day after the one picked for zones behind UTC:
toRRule deliberately writes the end of the chosen *local* day expressed
in UTC (the provider applies UNTIL coarsely), so the read side must
convert back before taking the date. Fixes the detail screen too, and
untilLocalDate is extracted so it can be tested.

Also: hoist a remember() out of a conditional (a slot that appears and
disappears breaks positional memoisation), match GroupedSurface's 22dp
corners instead of a drifted local 20dp copy, move the cards onto
GroupedSurface, drop the segmented row's icon slot so longer unit labels
fit, and reserve two lines so the stack stops shifting as the phrase
grows with each weekday.

Extract SettingsPrefs.firstDayOfWeek(scope), replacing three copies that
had drifted onto different initialValues.

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

app/src/main/java/de/jeanlucmakiola/calendula/data/prefs/SettingsPrefs.kt

@@ -23,8 +23,12 @@ import de.jeanlucmakiola.calendula.ui.common.IMPLEMENTED_VIEWS
 import de.jeanlucmakiola.calendula.ui.common.QuickSwitchConfig
 import de.jeanlucmakiola.calendula.ui.theme.FONT_SYSTEM_TOKEN
 import java.time.ZoneId
+import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.SharingStarted
+import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.flow.stateIn
 import kotlinx.datetime.DayOfWeek
 import java.time.temporal.WeekFields
 import java.util.Locale
@@ -82,6 +86,24 @@ fun WeekStartPref.resolveFirstDay(locale: Locale): DayOfWeek = when (this) {
     WeekStartPref.Auto -> DayOfWeek(WeekFields.of(locale).firstDayOfWeek.value)
 }
 
+/**
+ * The resolved first day of the week as a hot [StateFlow] — the one shape every
+ * screen that orders or lays out weekdays should use (the month grid, the agenda
+ * "this week" range, the recurrence weekday toggles). Sharing it keeps the
+ * initial-frame value identical everywhere; hand-rolled copies had drifted onto
+ * different `initialValue`s, so two surfaces could disagree for a frame.
+ */
+fun SettingsPrefs.firstDayOfWeek(scope: CoroutineScope): StateFlow<DayOfWeek> =
+    weekStart
+        .map { it.resolveFirstDay(Locale.getDefault()) }
+        .stateIn(
+            scope = scope,
+            started = SharingStarted.WhileSubscribed(5_000L),
+            // Seed with the locale convention rather than a hardcoded weekday, so
+            // the first frame is already right for everyone still on Auto.
+            initialValue = WeekStartPref.Auto.resolveFirstDay(Locale.getDefault()),
+        )
+
 /**
  * Display settings (M4) persisted app-side: theme override, Material You
  * dynamic colour, and week start. Language is handled separately through

app/src/main/java/de/jeanlucmakiola/calendula/ui/agenda/AgendaViewModel.kt

@@ -7,7 +7,7 @@ import de.jeanlucmakiola.calendula.data.calendar.CalendarRepository
 import de.jeanlucmakiola.calendula.data.di.IoDispatcher
 import de.jeanlucmakiola.calendula.data.prefs.PastEventDisplay
 import de.jeanlucmakiola.calendula.data.prefs.SettingsPrefs
-import de.jeanlucmakiola.calendula.data.prefs.resolveFirstDay
+import de.jeanlucmakiola.calendula.data.prefs.firstDayOfWeek
 import de.jeanlucmakiola.calendula.domain.CalendarSource
 import de.jeanlucmakiola.calendula.domain.EventInstance
 import de.jeanlucmakiola.calendula.domain.FailureReason
@@ -20,13 +20,11 @@ import kotlinx.coroutines.flow.catch
 import kotlinx.coroutines.flow.combine
 import kotlinx.coroutines.flow.flatMapLatest
 import kotlinx.coroutines.flow.flowOn
-import kotlinx.coroutines.flow.map
 import kotlinx.coroutines.flow.stateIn
 import kotlinx.datetime.DateTimeUnit
 import kotlinx.datetime.DayOfWeek
 import kotlinx.datetime.LocalDate
 import kotlinx.datetime.TimeZone
-import java.util.Locale
 import kotlinx.datetime.atStartOfDayIn
 import kotlinx.datetime.atTime
 import kotlinx.datetime.plus
@@ -51,8 +49,7 @@ class AgendaViewModel @Inject constructor(
     ) { range, showBar -> AgendaSettings(range, showBar) }
 
     // First day of the week, for the calendar-aligned "this week" range.
-    private val weekStartDay = settingsPrefs.weekStart
+    private val weekStartDay = settingsPrefs.firstDayOfWeek(viewModelScope)
-        .map { it.resolveFirstDay(Locale.getDefault()) }
 
     /**
      * How to treat events that already ended today (show / dim / hide). A display

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

@@ -9,6 +9,10 @@ import androidx.compose.ui.text.buildAnnotatedString
 import androidx.compose.ui.text.font.FontStyle
 import de.jeanlucmakiola.calendula.R
 import java.time.DayOfWeek
+import java.time.LocalDate
+import java.time.LocalDateTime
+import java.time.ZoneId
+import java.time.ZoneOffset
 import java.time.format.DateTimeFormatter
 import java.time.format.FormatStyle
 import java.time.format.TextStyle as JavaTextStyle
@@ -111,16 +115,36 @@ private fun rruleDayName(token: String, locale: Locale): String? {
 
 /** Parse an RRULE UNTIL value ("20261231" or "20261231T235959Z") to a localized date. */
 private fun parseUntilDate(raw: String, locale: Locale): String? {
-    val digits = raw.takeWhile { it.isDigit() }
+    val date = untilLocalDate(raw) ?: return null
-    if (digits.length < 8) return null
+    return DateTimeFormatter.ofLocalizedDate(FormatStyle.MEDIUM).withLocale(locale).format(date)
-    return try {
+}
-        val date = java.time.LocalDate.of(
+
+private val UNTIL_UTC_FORMAT: DateTimeFormatter =
+    DateTimeFormatter.ofPattern("yyyyMMdd'T'HHmmss'Z'", Locale.ROOT)
+
+/**
+ * The calendar day an RRULE UNTIL value denotes, *in the device's zone*.
+ *
+ * The UTC form ("20261231T225959Z") must be converted back before its date is
+ * read: `SimpleRecurrence.toRRule` writes the end of the chosen **local** day
+ * expressed in UTC, so for zones behind UTC that instant already falls on the
+ * following UTC date. Taking the leading digits straight off would then show
+ * the day after the one the user picked. Date-only and floating forms are
+ * already local and pass through unchanged.
+ */
+internal fun untilLocalDate(raw: String, zone: ZoneId = ZoneId.systemDefault()): LocalDate? {
+    val value = raw.trim()
+    return runCatching {
+        LocalDateTime.parse(value, UNTIL_UTC_FORMAT)
+            .atOffset(ZoneOffset.UTC)
+            .atZoneSameInstant(zone)
+            .toLocalDate()
+    }.recoverCatching {
+        val digits = value.takeWhile { it.isDigit() }
+        LocalDate.of(
             digits.substring(0, 4).toInt(),
             digits.substring(4, 6).toInt(),
             digits.substring(6, 8).toInt(),
         )
-        DateTimeFormatter.ofLocalizedDate(FormatStyle.MEDIUM).withLocale(locale).format(date)
+    }.getOrNull()
-    } catch (e: Exception) {
-        null
-    }
 }

app/src/main/java/de/jeanlucmakiola/calendula/ui/edit/EventEditScreen.kt

@@ -99,7 +99,6 @@ import androidx.compose.ui.text.style.TextOverflow
 import androidx.compose.ui.text.input.ImeAction
 import androidx.compose.ui.text.input.KeyboardCapitalization
 import androidx.compose.ui.text.input.KeyboardType
-import androidx.compose.ui.unit.Dp
 import androidx.compose.ui.unit.dp
 import androidx.core.content.ContextCompat
 import androidx.hilt.navigation.compose.hiltViewModel
@@ -140,11 +139,13 @@ import de.jeanlucmakiola.calendula.ui.common.eventFormFieldIcon
 import de.jeanlucmakiola.calendula.ui.common.eventFormFieldLabel
 import de.jeanlucmakiola.floret.components.FullScreenPicker
 import de.jeanlucmakiola.floret.components.GroupedRow
+import de.jeanlucmakiola.floret.components.GroupedSurface
 import de.jeanlucmakiola.calendula.ui.common.MILLIS_PER_DAY
 import de.jeanlucmakiola.floret.components.InlineTextField
 import de.jeanlucmakiola.floret.components.OptionCard
 import de.jeanlucmakiola.floret.components.OptionPicker
 import de.jeanlucmakiola.floret.components.Position
+import de.jeanlucmakiola.floret.components.groupedShape as floretGroupedShape
 import de.jeanlucmakiola.floret.components.positionOf
 import de.jeanlucmakiola.calendula.ui.common.REMINDER_PRESETS
 import de.jeanlucmakiola.calendula.ui.common.TimePickerAlert
@@ -1328,8 +1329,21 @@ private fun RecurrencePickerDialog(
 
     val locale = currentLocale()
     val untilDate = untilIso?.let { runCatching { LocalDate.parse(it) }.getOrNull() }
-    val interval = intervalText.toIntOrNull()?.takeIf { it in 1..999 }
+    // remember() must not sit behind the ?. — a slot that appears and disappears
-    val count = countText.toIntOrNull()?.takeIf { it in 1..999 }
+    // as a date is picked breaks Compose's positional memoisation. Format
+    // unconditionally and let the null fall through to a null summary.
+    val untilSummary = remember(untilDate, locale) {
+        untilDate?.let {
+            DateTimeFormatter.ofLocalizedDate(FormatStyle.MEDIUM)
+                .withLocale(locale).format(it.toJavaLocalDate())
+        }
+    }
+    // A blank amount field shows its placeholder ("1" / "10") greyed out, so read
+    // blank as that default rather than as an error — otherwise simply clearing
+    // the field silently greys out OK. Only a real out-of-range value (0) is
+    // invalid, and that's the one case the read-out below reports.
+    val interval = if (intervalText.isBlank()) 1 else intervalText.toIntOrNull()?.takeIf { it in 1..999 }
+    val count = if (countText.isBlank()) 10 else countText.toIntOrNull()?.takeIf { it in 1..999 }
     val customEnd: RecurrenceEnd? = when (endMode) {
         RecurrenceEndMode.Never -> RecurrenceEnd.Never
         RecurrenceEndMode.Until -> untilDate?.let { RecurrenceEnd.Until(it) }
@@ -1387,22 +1401,25 @@ private fun RecurrencePickerDialog(
             // a parent arrangement would leave a residual gap that snaps shut.
             Column(modifier = Modifier.fillMaxWidth()) {
                 // A live, human-readable read-out of the rule being built, so the
-                // effect of the controls below is never a guess.
+                // effect of the controls below is never a guess. It renders
+                // *[customResult] itself* — the exact string OK would save — so it
+                // can never describe a different rule than the one that lands. When
+                // the form can't produce a rule, it says why instead of falling
+                // back to defaults and quietly disagreeing with the controls.
+                // minLines reserves the second line up front: without it the whole
+                // stack below shifts a line as the phrase grows with each weekday.
                 Text(
-                    text = recurrenceText(
+                    text = customResult?.let { recurrenceText(it, locale) }
-                        SimpleRecurrence(
+                        ?: AnnotatedString(
-                            freq = freq,
+                            stringResource(R.string.event_edit_recurrence_incomplete),
-                            interval = interval ?: 1,
+                        ),
-                            end = customEnd ?: RecurrenceEnd.Never,
-                            byDays = if (freq == RecurrenceFreq.Weekly) {
-                                daysMask.toDaySet()
-                            } else {
-                                emptySet()
-                            },
-                        ).toRRule(),
-                        locale,
-                    ),
                     style = MaterialTheme.typography.titleMedium,
+                    color = if (customResult != null) {
+                        MaterialTheme.colorScheme.onSurface
+                    } else {
+                        MaterialTheme.colorScheme.error
+                    },
+                    minLines = 2,
                     modifier = Modifier
                         .fillMaxWidth()
                         .padding(horizontal = 16.dp),
@@ -1410,11 +1427,9 @@ private fun RecurrencePickerDialog(
 
                 // How often: an interval amount plus a frequency segmented row —
                 // all four units on show, no unit hidden behind a dropdown.
-                Surface(
+                GroupedSurface(
-                    color = MaterialTheme.colorScheme.surfaceContainerHigh,
+                    position = Position.Alone,
-                    shape = groupedShape(Position.Alone),
                     modifier = Modifier
-                        .fillMaxWidth()
                         .padding(top = 16.dp)
                         .padding(horizontal = 16.dp),
                 ) {
@@ -1442,7 +1457,18 @@ private fun RecurrencePickerDialog(
                                     shape = SegmentedButtonDefaults.itemShape(
                                         index, RecurrenceFreq.entries.size,
                                     ),
-                                    label = { Text(stringResource(recurrenceUnitLabel(entry))) },
+                                    // Drop the default check-icon slot: four segments
+                                    // share one row, and its reserved width is what
+                                    // pushes longer labels ("Monate", "Wochen") into
+                                    // an ellipsis on a compact screen. Selection is
+                                    // already carried by the segment's fill.
+                                    icon = {},
+                                    label = {
+                                        Text(
+                                            text = stringResource(recurrenceUnitLabel(entry)),
+                                            maxLines = 1,
+                                        )
+                                    },
                                 )
                             }
                         }
@@ -1452,11 +1478,9 @@ private fun RecurrencePickerDialog(
                 // Weekday picks — only meaningful on a weekly rule. The 16dp gap
                 // sits inside the animated block so it grows/shrinks with the card.
                 AnimatedVisibility(visible = freq == RecurrenceFreq.Weekly) {
-                    Surface(
+                    GroupedSurface(
-                        color = MaterialTheme.colorScheme.surfaceContainerHigh,
+                        position = Position.Alone,
-                        shape = groupedShape(Position.Alone),
                         modifier = Modifier
-                            .fillMaxWidth()
                             .padding(top = 16.dp)
                             .padding(horizontal = 16.dp),
                     ) {
@@ -1487,12 +1511,7 @@ private fun RecurrencePickerDialog(
                     )
                     GroupedRow(
                         title = stringResource(R.string.event_edit_recurrence_end_until),
-                        summary = untilDate?.let {
+                        summary = untilSummary,
-                            remember(it, locale) {
-                                DateTimeFormatter.ofLocalizedDate(FormatStyle.MEDIUM)
-                                    .withLocale(locale).format(it.toJavaLocalDate())
-                            }
-                        },
                         position = Position.Middle,
                         selected = endMode == RecurrenceEndMode.Until,
                         onClick = {
@@ -1510,13 +1529,13 @@ private fun RecurrencePickerDialog(
                         selected = endMode == RecurrenceEndMode.Count,
                         onClick = { endMode = RecurrenceEndMode.Count },
                     )
-                    if (endMode == RecurrenceEndMode.Count) {
+                    // Animated for the same reason the weekday card is: this is the
-                        Surface(
+                    // picker's other collapsible block, and a bare `if` here made
-                            color = MaterialTheme.colorScheme.surfaceContainerHigh,
+                    // the ends group snap while the card above it glided.
-                            shape = groupedShape(Position.Bottom),
+                    AnimatedVisibility(visible = endMode == RecurrenceEndMode.Count) {
-                            modifier = Modifier
+                        GroupedSurface(
-                                .fillMaxWidth()
+                            position = Position.Bottom,
-                                .padding(horizontal = 16.dp),
+                            modifier = Modifier.padding(horizontal = 16.dp),
                         ) {
                             Row(
                                 verticalAlignment = Alignment.CenterVertically,
@@ -1549,7 +1568,13 @@ private fun RecurrencePickerDialog(
                 untilIso = it.toString()
                 showUntilPicker = false
             },
-            onDismiss = { showUntilPicker = false },
+            onDismiss = {
+                showUntilPicker = false
+                // Backing out of the date picker with nothing chosen would leave
+                // "on date" selected but dateless — a state that can't be saved and
+                // greys out OK with no visible cause. Fall back to "never".
+                if (untilIso == null) endMode = RecurrenceEndMode.Never
+            },
         )
     }
 }
@@ -1625,18 +1650,14 @@ private fun recurrenceUnitLabel(freq: RecurrenceFreq): Int = when (freq) {
     RecurrenceFreq.Yearly -> R.string.recurrence_unit_years
 }
 
-/** Corner shape for a card at [position] in a grouped run (mirrors GroupedRow). */
+/**
-private fun groupedShape(position: Position, full: Dp = 20.dp, small: Dp = 6.dp): Shape =
+ * Corner shape for a card at [position] in a grouped run. Delegates to the
-    when (position) {
+ * family primitive with the same radii [GroupedSurface] draws at rest — a local
-        Position.Alone -> RoundedCornerShape(full)
+ * copy had drifted to 20dp, so a card sharing a run with GroupedRows had
-        Position.Top -> RoundedCornerShape(
+ * visibly tighter outer corners than the rows above it.
-            topStart = full, topEnd = full, bottomStart = small, bottomEnd = small,
+ */
-        )
+private fun groupedShape(position: Position): Shape =
-        Position.Middle -> RoundedCornerShape(small)
+    floretGroupedShape(position, full = 22.dp, small = 6.dp)
-        Position.Bottom -> RoundedCornerShape(
-            topStart = small, topEnd = small, bottomStart = full, bottomEnd = full,
-        )
-    }
 
 /** The 2dp gap that visually separates grouped cards (none after the last). */
 private fun groupedGap(position: Position): Modifier = when (position) {

app/src/main/java/de/jeanlucmakiola/calendula/ui/edit/EventEditViewModel.kt

@@ -8,9 +8,8 @@ import de.jeanlucmakiola.calendula.data.calendar.NoSuchEventException
 import de.jeanlucmakiola.calendula.data.di.IoDispatcher
 import de.jeanlucmakiola.calendula.data.prefs.CalendarPrefs
 import de.jeanlucmakiola.calendula.data.prefs.SettingsPrefs
-import de.jeanlucmakiola.calendula.data.prefs.WeekStartPref
+import de.jeanlucmakiola.calendula.data.prefs.firstDayOfWeek
 import de.jeanlucmakiola.calendula.data.prefs.resolveDefaultReminder
-import de.jeanlucmakiola.calendula.data.prefs.resolveFirstDay
 import de.jeanlucmakiola.calendula.domain.AccessLevel
 import de.jeanlucmakiola.calendula.domain.Availability
 import de.jeanlucmakiola.calendula.domain.CalendarSource
@@ -49,7 +48,6 @@ import kotlinx.datetime.LocalTime
 import kotlinx.datetime.TimeZone
 import kotlinx.datetime.toInstant
 import kotlinx.datetime.toLocalDateTime
-import java.util.Locale
 import kotlin.coroutines.cancellation.CancellationException
 import kotlin.time.Clock
 import kotlin.time.Duration.Companion.hours
@@ -276,13 +274,7 @@ class EventEditViewModel @Inject constructor(
      * app's "week starts on" preference, matching the calendar views (Auto
      * falls back to the locale convention).
      */
-    val firstDayOfWeek: StateFlow<DayOfWeek> = settingsPrefs.weekStart
+    val firstDayOfWeek: StateFlow<DayOfWeek> = settingsPrefs.firstDayOfWeek(viewModelScope)
-        .map { it.resolveFirstDay(Locale.getDefault()) }
-        .stateIn(
-            scope = viewModelScope,
-            started = SharingStarted.WhileSubscribed(5_000L),
-            initialValue = WeekStartPref.Auto.resolveFirstDay(Locale.getDefault()),
-        )
 
     /**
      * Initialise a fresh form for a new event on [date]. [startMinutes] (minutes

app/src/main/java/de/jeanlucmakiola/calendula/ui/month/MonthViewModel.kt

@@ -6,7 +6,7 @@ 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.prefs.SettingsPrefs
-import de.jeanlucmakiola.calendula.data.prefs.resolveFirstDay
+import de.jeanlucmakiola.calendula.data.prefs.firstDayOfWeek
 import de.jeanlucmakiola.calendula.domain.CalendarSource
 import de.jeanlucmakiola.calendula.domain.EventInstance
 import de.jeanlucmakiola.calendula.domain.FailureReason
@@ -21,7 +21,6 @@ import kotlinx.coroutines.flow.catch
 import kotlinx.coroutines.flow.combine
 import kotlinx.coroutines.flow.flatMapLatest
 import kotlinx.coroutines.flow.flowOn
-import kotlinx.coroutines.flow.map
 import kotlinx.coroutines.flow.stateIn
 import kotlinx.datetime.DateTimeUnit
 import kotlinx.datetime.DayOfWeek
@@ -34,7 +33,6 @@ import kotlinx.datetime.minus
 import kotlinx.datetime.plus
 import kotlinx.datetime.toInstant
 import kotlinx.datetime.toLocalDateTime
-import java.util.Locale
 import kotlin.time.Clock
 import kotlin.time.Instant
 import javax.inject.Inject
@@ -48,16 +46,9 @@ class MonthViewModel @Inject constructor(
 ) : ViewModel() {
 
     private val zone = TimeZone.currentSystemDefault()
-    private val locale: Locale = Locale.getDefault()
 
     /** First day of the week, from the Settings preference (AUTO → locale). */
-    val weekStart: StateFlow<DayOfWeek> = settingsPrefs.weekStart
+    val weekStart: StateFlow<DayOfWeek> = settingsPrefs.firstDayOfWeek(viewModelScope)
-        .map { it.resolveFirstDay(locale) }
-        .stateIn(
-            scope = viewModelScope,
-            started = SharingStarted.WhileSubscribed(5_000L),
-            initialValue = DayOfWeek.MONDAY,
-        )
 
     /** Whether to fade events that have already finished (display concern only). */
     val dimCompletedEvents: StateFlow<Boolean> = settingsPrefs.dimCompletedEvents

app/src/main/res/values/strings.xml

@@ -149,6 +149,9 @@
     <string name="recurrence_unit_weeks">weeks</string>
     <string name="recurrence_unit_months">months</string>
     <string name="recurrence_unit_years">years</string>
+    <!-- Shown in place of the live rule read-out when an amount field holds a
+         value outside 1–999 (i.e. 0), so the greyed-out OK button has a reason. -->
+    <string name="event_edit_recurrence_incomplete">Enter a number from 1 to 999</string>
     <string name="event_edit_recurrence_ends">Ends</string>
     <string name="event_edit_recurrence_end_never">Never</string>
     <string name="event_edit_recurrence_end_until">On a date</string>

app/src/test/java/de/jeanlucmakiola/calendula/ui/common/RecurrenceTextTest.kt

@@ -0,0 +1,69 @@
+package de.jeanlucmakiola.calendula.ui.common
+
+import com.google.common.truth.Truth.assertThat
+import java.time.LocalDate
+import java.time.ZoneId
+import org.junit.jupiter.api.Test
+
+/**
+ * The read side of the UNTIL round-trip. `SimpleRecurrence.toRRule` deliberately
+ * writes the end of the chosen *local* day expressed in UTC (see its docs — the
+ * provider applies UNTIL coarsely), so displaying it means converting back into
+ * the device zone first. Reading the leading digits raw showed the wrong day for
+ * zones behind UTC.
+ */
+class RecurrenceTextTest {
+
+    private val berlin = ZoneId.of("Europe/Berlin")
+    private val losAngeles = ZoneId.of("America/Los_Angeles")
+    private val utc = ZoneId.of("UTC")
+
+    @Test
+    fun `UTC form converts back to the picked day west of UTC`() {
+        // toRRule("2026-12-31", America/Los_Angeles) → 23:59:59 local = 07:59:59Z
+        // on 1 Jan. Reading the digits raw would show 1 Jan 2027.
+        assertThat(untilLocalDate("20270101T075959Z", losAngeles))
+            .isEqualTo(LocalDate.of(2026, 12, 31))
+    }
+
+    @Test
+    fun `UTC form converts back to the picked day east of UTC`() {
+        // Berlin in December is UTC+1: 23:59:59 local = 22:59:59Z the same day.
+        assertThat(untilLocalDate("20261231T225959Z", berlin))
+            .isEqualTo(LocalDate.of(2026, 12, 31))
+    }
+
+    @Test
+    fun `UTC form is unchanged at UTC itself`() {
+        assertThat(untilLocalDate("20261231T235959Z", utc))
+            .isEqualTo(LocalDate.of(2026, 12, 31))
+    }
+
+    @Test
+    fun `summer offset is honoured, not a fixed one`() {
+        // Berlin in July is UTC+2, so the same local end-of-day lands at 21:59:59Z.
+        assertThat(untilLocalDate("20260801T215959Z", berlin))
+            .isEqualTo(LocalDate.of(2026, 8, 1))
+    }
+
+    @Test
+    fun `date-only form is already local and passes through`() {
+        assertThat(untilLocalDate("20261231", losAngeles))
+            .isEqualTo(LocalDate.of(2026, 12, 31))
+    }
+
+    @Test
+    fun `floating date-time form uses its date as-is`() {
+        // No trailing Z, so it isn't an instant — no conversion may be applied.
+        assertThat(untilLocalDate("20261231T235959", losAngeles))
+            .isEqualTo(LocalDate.of(2026, 12, 31))
+    }
+
+    @Test
+    fun `garbage returns null rather than throwing`() {
+        assertThat(untilLocalDate("", berlin)).isNull()
+        assertThat(untilLocalDate("nonsense", berlin)).isNull()
+        assertThat(untilLocalDate("2026", berlin)).isNull()
+        assertThat(untilLocalDate("20261340", berlin)).isNull()
+    }
+}