Merge branch 'feat/24-automatic-home-clock' into release/v1.0.0

# Conflicts:
#	CHANGELOG.md
#	app/src/main/java/de/jeanlucmakiola/clockula/data/backup/BackupCodec.kt
#	app/src/main/java/de/jeanlucmakiola/clockula/data/backup/BackupDto.kt
#	app/src/main/java/de/jeanlucmakiola/clockula/data/prefs/ClockPrefs.kt
#	app/src/main/java/de/jeanlucmakiola/clockula/data/prefs/SettingsPrefs.kt
#	app/src/main/java/de/jeanlucmakiola/clockula/domain/ClockDefaults.kt
#	app/src/main/java/de/jeanlucmakiola/clockula/ui/settings/ClockSettingsScreen.kt
#	app/src/main/java/de/jeanlucmakiola/clockula/ui/worldclock/WorldClockRow.kt
#	app/src/main/java/de/jeanlucmakiola/clockula/ui/worldclock/WorldClockRowState.kt
#	app/src/main/java/de/jeanlucmakiola/clockula/ui/worldclock/WorldClockSource.kt
#	app/src/main/res/values/strings.xml
#	app/src/test/java/de/jeanlucmakiola/clockula/data/backup/BackupCodecTest.kt
#	app/src/test/java/de/jeanlucmakiola/clockula/data/prefs/ClockPrefsTest.kt
#	app/src/test/java/de/jeanlucmakiola/clockula/data/prefs/SettingsPrefsTest.kt
#	docs/ARCHITECTURE.md
#	docs/BACKUP.md

app/src/main/java/de/jeanlucmakiola/clockula/data/backup/BackupCodec.kt

@@ -143,6 +143,7 @@ object BackupCodec {
         alarmSilenceAfterMinutes = defaults.alarmSilenceAfterMinutes,
         timerSilenceAfterMinutes = defaults.timerSilenceAfterMinutes,
         volumeButtons = defaults.volumeButtons.name,
+        automaticLocalClock = defaults.automaticLocalClock,
         worldClockSeconds = worldClockSeconds,
         timerPresets = timerPresets.map { it.inWholeMilliseconds },
         bedtime = bedtime?.toDto(),
@@ -300,6 +301,7 @@ object BackupCodec {
                 alarmSilenceAfterMinutes = settingsDto.alarmSilenceAfterMinutes,
                 timerSilenceAfterMinutes = settingsDto.timerSilenceAfterMinutes,
                 volumeButtons = settingsDto.volumeButtons.toEnum(VolumeButtonAction.CONTROL_VOLUME),
+                automaticLocalClock = settingsDto.automaticLocalClock,
             ),
             timerPresets = settingsDto.timerPresets?.map { it.milliseconds } ?: TimerPresets.BUILT_IN,
             worldClockSeconds = settingsDto.worldClockSeconds,

app/src/main/java/de/jeanlucmakiola/clockula/data/backup/BackupDto.kt

@@ -93,6 +93,8 @@ data class SettingsDto(
      */
     @SerialName("default_alarm_silence_after_minutes") val alarmSilenceAfterMinutes: Int = SilenceAfter.DEFAULT_MINUTES,
     @SerialName("default_timer_silence_after_minutes") val timerSilenceAfterMinutes: Int = SilenceAfter.DEFAULT_MINUTES,
+    /** Absent in files from before #24, which keeps the default: on. */
+    @SerialName("automatic_local_clock") val automaticLocalClock: Boolean = true,
     /** Absent in files from before it existed, which keeps the default: off. */
     @SerialName("world_clock_show_seconds") val worldClockSeconds: Boolean = false,
     /**

app/src/main/java/de/jeanlucmakiola/clockula/data/prefs/ClockPrefs.kt

@@ -74,6 +74,13 @@ object ClockPrefs {
      */
     val worldClockSeconds: Pref<Boolean> = booleanPref("world_clock_show_seconds", false)
 
+    /**
+     * The World clock tab's temporary local-time row while the device is in a
+     * zone whose time differs from home (#24). On by default, as Google Clock's
+     * automatic home clock is.
+     */
+    val automaticLocalClock: Pref<Boolean> = booleanPref("automatic_local_clock", true)
+
     private fun Pref<Int>.clampedTo(range: IntRange): Pref<Int> =
         map(decode = { it.coerceIn(range) }, encode = { it.coerceIn(range) })
 

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

@@ -79,9 +79,14 @@ class SettingsPrefs @Inject constructor(private val store: PrefStore) {
         ) { timerRingtoneUri, timerVibrate, timerVolumeRampSeconds ->
             Triple(timerRingtoneUri, timerVibrate, timerVolumeRampSeconds)
         },
-        store.flow(ClockPrefs.alarmRingtoneUri),
-        store.flow(ClockPrefs.homeZoneId),
-    ) { partial, (alarmSilenceAfter, timerSilenceAfter, volumeButtons), (timerRingtoneUri, timerVibrate, timerVolumeRampSeconds), alarmRingtoneUri, homeZoneId ->
+        combine(
+            store.flow(ClockPrefs.alarmRingtoneUri),
+            store.flow(ClockPrefs.homeZoneId),
+            store.flow(ClockPrefs.automaticLocalClock),
+        ) { alarmRingtoneUri, homeZoneId, automaticLocalClock ->
+            Triple(alarmRingtoneUri, homeZoneId, automaticLocalClock)
+        },
+    ) { partial, (alarmSilenceAfter, timerSilenceAfter, volumeButtons), (timerRingtoneUri, timerVibrate, timerVolumeRampSeconds), (alarmRingtoneUri, homeZoneId, automaticLocalClock) ->
         partial.copy(
             alarmRingtoneUri = alarmRingtoneUri,
             timerRingtoneUri = timerRingtoneUri,
@@ -91,6 +96,7 @@ class SettingsPrefs @Inject constructor(private val store: PrefStore) {
             alarmSilenceAfterMinutes = alarmSilenceAfter,
             timerSilenceAfterMinutes = timerSilenceAfter,
             volumeButtons = volumeButtons,
+            automaticLocalClock = automaticLocalClock,
         )
     }.distinctUntilChanged()
 
@@ -118,6 +124,7 @@ class SettingsPrefs @Inject constructor(private val store: PrefStore) {
             prefs[ClockPrefs.alarmSilenceAfterMinutes] = backup.defaults.alarmSilenceAfterMinutes
             prefs[ClockPrefs.timerSilenceAfterMinutes] = backup.defaults.timerSilenceAfterMinutes
             prefs[ClockPrefs.volumeButtons] = backup.defaults.volumeButtons
+            prefs[ClockPrefs.automaticLocalClock] = backup.defaults.automaticLocalClock
             prefs[ClockPrefs.worldClockSeconds] = backup.worldClockSeconds
             prefs[TimerPrefs.presets] = backup.timerPresets
             // Absent from a file written before bedtime existed: keep it.
@@ -169,6 +176,9 @@ class SettingsPrefs @Inject constructor(private val store: PrefStore) {
 
     suspend fun setHomeZoneId(zoneId: String?) = store.set(ClockPrefs.homeZoneId, zoneId)
 
+    suspend fun setAutomaticLocalClock(enabled: Boolean) =
+        store.set(ClockPrefs.automaticLocalClock, enabled)
+
     // --- world clock display (#23) ---
 
     /** Its own flow, distinct-until-changed, so the clock defaults' watchers never see it move. */

app/src/main/java/de/jeanlucmakiola/clockula/domain/ClockDefaults.kt

@@ -33,6 +33,8 @@ data class ClockDefaults(
     val alarmSilenceAfterMinutes: Int = SilenceAfter.DEFAULT_MINUTES,
     val timerSilenceAfterMinutes: Int = SilenceAfter.DEFAULT_MINUTES,
     val volumeButtons: VolumeButtonAction = VolumeButtonAction.CONTROL_VOLUME,
+    /** The World clock tab's temporary local-time row while travelling (#24). */
+    val automaticLocalClock: Boolean = true,
 ) {
     /** How long a ringing alarm rings before it is silenced as missed. */
     val alarmSilenceAfter: Duration get() = SilenceAfter.window(alarmSilenceAfterMinutes)

app/src/main/java/de/jeanlucmakiola/clockula/domain/worldclock/WorldClocks.kt

@@ -2,6 +2,7 @@ package de.jeanlucmakiola.clockula.domain.worldclock
 
 import de.jeanlucmakiola.clockula.domain.Zones
 import java.time.ZoneId
+import kotlin.time.Instant
 
 object WorldClocks {
     /**
@@ -39,3 +40,48 @@ object HomeZone {
             ?.let { runCatching { ZoneId.of(it) }.getOrNull() }
             ?: deviceZone
 }
+
+/**
+ * The automatic local clock (#24): the device's own zone as a temporary row
+ * while it is somewhere the time differs from home.
+ *
+ * It is Google Clock's "automatic home clock" turned round. Google's hero is
+ * local time and adds home as a row; Clockula's hero face *is* home, so home is
+ * already on screen while travelling and the clock that goes missing is the
+ * local one.
+ */
+object LocalClock {
+
+    /**
+     * [deviceZone] when the tab should show it as a temporary row, else null.
+     *
+     * "Different" means a different UTC offset **at [at]**, as Google decides
+     * it: Paris while home is Berlin shows no row, and neither does London in
+     * winter while home is Lisbon. It is asked again on every tick, so a DST
+     * change on either side moves it with the clock.
+     *
+     * [homeZone] is `HomeZone.resolve`'s answer. With no home stored it *is*
+     * [deviceZone], so a home that follows the device never shows a row.
+     *
+     * Null when [enabled] is off, when [deviceZone] is not a zone id the
+     * tzdata lists (a bare offset, which no row could be read against), and
+     * when [savedZoneIds] already holds a known zone at the device's offset:
+     * that row already shows the local time, and a second one would be a
+     * duplicate.
+     */
+    fun zoneToShow(
+        enabled: Boolean,
+        homeZone: ZoneId,
+        deviceZone: ZoneId,
+        savedZoneIds: List<String>,
+        at: Instant,
+    ): ZoneId? {
+        if (!enabled || !Zones.isValid(deviceZone.id)) return null
+        val deviceOffset = ZoneComparisons.utcOffsetMinutes(deviceZone, at)
+        if (ZoneComparisons.utcOffsetMinutes(homeZone, at) == deviceOffset) return null
+        val alreadyShown = savedZoneIds.any { id ->
+            Zones.isValid(id) && ZoneComparisons.utcOffsetMinutes(ZoneId.of(id), at) == deviceOffset
+        }
+        return deviceZone.takeUnless { alreadyShown }
+    }
+}

app/src/main/java/de/jeanlucmakiola/clockula/domain/worldclock/ZoneCatalog.kt

@@ -57,13 +57,32 @@ object ZoneCatalog {
 
     /**
      * The last `/`-segment with underscores replaced by spaces, trimmed; the
-     * input trimmed when that is blank. Never throws.
+     * input trimmed when that is blank. `Etc/GMT±n` is the exception: it is
+     * named by its real offset instead. Never throws.
      */
     fun cityFallbackFor(zoneId: String): String {
+        etcOffsetName(zoneId)?.let { return it }
         val segment = zoneId.substringAfterLast('/').replace('_', ' ').trim()
         return segment.ifBlank { zoneId.trim() }
     }
 
+    /**
+     * `Etc/GMT-9` is UTC**+**9: POSIX signs these ids the other way round, so
+     * their last segment would name the zone with the wrong sign. A device set
+     * by UTC offset rather than by city (Android 13+ offers it) reports one, so
+     * the face and the local row name it by its real offset ("GMT+9").
+     */
+    private fun etcOffsetName(zoneId: String): String? {
+        val match = ETC_OFFSET.matchEntire(zoneId.trim()) ?: return null
+        val hours = match.groupValues[2].toInt()
+        val sign = if (match.groupValues[1] == "-") 1 else -1
+        return ZoneOffsetFormat.gmtLabel(sign * hours * MINUTES_PER_HOUR)
+    }
+
+    private const val MINUTES_PER_HOUR = 60
+
+    private val ETC_OFFSET = Regex("""Etc/GMT([+-])(\d{1,2})""")
+
     /** By `ZoneSearch.fold(city)` ascending, then [ZoneEntry.zoneId] ascending. */
     fun sorted(entries: List<ZoneEntry>): List<ZoneEntry> =
         entries.sortedWith(compareBy({ ZoneSearch.fold(it.city) }, { it.zoneId }))

app/src/main/java/de/jeanlucmakiola/clockula/ui/settings/ClockSettingsScreen.kt

@@ -35,7 +35,7 @@ internal fun ClockSettingsScreen(
         GroupedRow(
             title = stringResource(R.string.settings_world_clock_seconds),
             summary = stringResource(R.string.settings_world_clock_seconds_hint),
-            position = Position.Bottom,
+            position = Position.Middle,
             trailing = {
                 Switch(
                     checked = state.worldClockSeconds,
@@ -44,5 +44,29 @@ internal fun ClockSettingsScreen(
             },
             onClick = { viewModel.onWorldClockSecondsChanged(!state.worldClockSeconds) },
         )
+        // Home that follows the device is always local time, so the row this
+        // switches can never appear. The switch stays live (the choice is
+        // remembered for when a home is set) and the hint says why it is idle.
+        val followsDevice = state.defaults.homeZoneId == null
+        val automaticLocalClock = state.defaults.automaticLocalClock
+        GroupedRow(
+            title = stringResource(R.string.settings_automatic_local_clock),
+            summary = stringResource(
+                if (followsDevice) {
+                    R.string.settings_automatic_local_clock_needs_home
+                } else {
+                    R.string.settings_automatic_local_clock_hint
+                },
+            ),
+            position = Position.Bottom,
+            dimmed = followsDevice,
+            trailing = {
+                Switch(
+                    checked = automaticLocalClock,
+                    onCheckedChange = viewModel::onAutomaticLocalClockChanged,
+                )
+            },
+            onClick = { viewModel.onAutomaticLocalClockChanged(!automaticLocalClock) },
+        )
     }
 }

app/src/main/java/de/jeanlucmakiola/clockula/ui/settings/SettingsViewModel.kt

@@ -282,6 +282,9 @@ class SettingsViewModel @Inject internal constructor(
         viewModelScope.launch { settings.setHomeZoneId(null) }
     }
 
+    fun onAutomaticLocalClockChanged(value: Boolean) =
+        viewModelScope.launch { settings.setAutomaticLocalClock(value) }
+
     private companion object {
         const val SUBSCRIPTION_GRACE_MS = 5_000L
     }

app/src/main/java/de/jeanlucmakiola/clockula/ui/worldclock/WorldClockRow.kt

@@ -3,13 +3,16 @@ package de.jeanlucmakiola.clockula.ui.worldclock
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.RowScope
 import androidx.compose.foundation.layout.fillMaxWidth
 import androidx.compose.foundation.layout.fillMaxHeight
+import androidx.compose.foundation.layout.heightIn
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.size
 import androidx.compose.foundation.layout.widthIn
 import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.rounded.DragHandle
+import androidx.compose.material.icons.rounded.NearMe
 import androidx.compose.material3.Icon
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Text
@@ -18,6 +21,7 @@ import androidx.compose.runtime.Immutable
 import androidx.compose.runtime.remember
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.platform.LocalConfiguration
 import androidx.compose.ui.platform.LocalDensity
 import androidx.compose.ui.res.pluralStringResource
@@ -134,45 +138,17 @@ internal fun WorldClockRow(
                 tint = MaterialTheme.colorScheme.onSurfaceVariant,
                 modifier = dragHandle.size(24.dp),
             )
-            Column(modifier = Modifier.weight(1f)) {
-                Text(
-                    text = row.name,
-                    style = MaterialTheme.typography.bodyLarge,
-                    color = MaterialTheme.colorScheme.onSurface,
-                    maxLines = 1,
-                    overflow = TextOverflow.Ellipsis,
-                )
-                Text(
-                    text = summary,
-                    style = MaterialTheme.typography.bodySmall,
-                    color = MaterialTheme.colorScheme.onSurfaceVariant,
-                    maxLines = 1,
-                    overflow = TextOverflow.Ellipsis,
-                )
-            }
-            if (time != null) {
-                Text(
-                    text = time,
-                    // Tabular, because M7 made the headline role tabular
-                    // app-wide: a column of times must not jitter as it ticks.
-                    style = timeStyle.style,
-                    // One line only when capped (seconds on); without seconds
-                    // the time wraps at a huge font scale, as it always did.
-                    maxLines = if (timeStyle.capped) 1 else Int.MAX_VALUE,
-                    softWrap = !timeStyle.capped,
-                    overflow = if (timeStyle.capped) TextOverflow.Ellipsis else TextOverflow.Clip,
-                    // The row's description already says the time, to the
-                    // minute; the ticking text itself stays out of the tree.
-                    modifier = Modifier
-                        .widthIn(max = timeStyle.maxWidth)
-                        .clearAndSetSemantics {},
-                    color = if (isDragging) {
-                        MaterialTheme.colorScheme.primary
-                    } else {
-                        MaterialTheme.colorScheme.onSurface
-                    },
-                )
-            }
+            RowText(
+                name = row.name,
+                summary = summary,
+                time = time,
+                timeStyle = timeStyle,
+                timeColor = if (isDragging) {
+                    MaterialTheme.colorScheme.primary
+                } else {
+                    MaterialTheme.colorScheme.onSurface
+                },
+            )
         }
     }
 }
@@ -234,6 +210,108 @@ internal fun rememberRowTimeStyle(showSeconds: Boolean, formatTime: (LocalTime)
     }
 }
 
+/**
+ * The automatic local clock (#24): the device's own zone while it is somewhere
+ * the time differs from home. Drawn like a city row but outside the reorderable
+ * column, with a location glyph where the drag handle would be and no actions
+ * at all — it is not stored, so there is nothing to move or remove, and it goes
+ * away by itself once the device is back on home time.
+ */
+@Composable
+internal fun LocalClockRow(
+    row: WorldClockRowState,
+    formatTime: (LocalTime) -> String,
+    formatSpokenTime: (LocalTime) -> String,
+    timeStyle: RowTimeStyle,
+    modifier: Modifier = Modifier,
+) {
+    // "Local time · 7 hours ahead": the label is what tells it apart from a
+    // saved city, so it leads the summary rather than hiding in a glyph.
+    val summary = stringResource(R.string.world_clock_summary, stringResource(R.string.world_clock_local), summaryFor(row))
+    val time = row.time?.let(formatTime)
+    // `LocalClock` only offers a zone the device lists, so the time is there;
+    // the fallback keeps TalkBack from reading an empty slot if that changes.
+    // Spoken to the minute, as a saved row is.
+    val spokenTime = row.time?.let(formatSpokenTime)
+    val description = if (spokenTime != null) {
+        stringResource(R.string.world_clock_row_description, row.name, spokenTime, summary)
+    } else {
+        stringResource(R.string.world_clock_row_description_unknown, row.name, summary)
+    }
+
+    GroupedSurface(
+        position = Position.Alone,
+        modifier = modifier.padding(horizontal = GroupedListInset),
+    ) {
+        Row(
+            modifier = Modifier
+                .fillMaxWidth()
+                .heightIn(min = 80.dp)
+                .padding(start = 12.dp, end = 20.dp, top = 8.dp, bottom = 8.dp)
+                .semantics(mergeDescendants = true) { contentDescription = description },
+            horizontalArrangement = Arrangement.spacedBy(12.dp),
+            verticalAlignment = Alignment.CenterVertically,
+        ) {
+            Icon(
+                imageVector = Icons.Rounded.NearMe,
+                contentDescription = null,
+                tint = MaterialTheme.colorScheme.primary,
+                modifier = Modifier.size(24.dp),
+            )
+            // Two lines, not one: "Local time · " is prepended, and on a phone
+            // the day ("Tomorrow") is what an ellipsis would otherwise eat.
+            RowText(name = row.name, summary = summary, time = time, timeStyle = timeStyle, summaryMaxLines = 2)
+        }
+    }
+}
+
+/** The name over the summary, then the time: what a saved row and the local row share. */
+@Composable
+private fun RowScope.RowText(
+    name: String,
+    summary: String,
+    time: String?,
+    timeStyle: RowTimeStyle,
+    timeColor: Color = MaterialTheme.colorScheme.onSurface,
+    summaryMaxLines: Int = 1,
+) {
+    Column(modifier = Modifier.weight(1f)) {
+        Text(
+            text = name,
+            style = MaterialTheme.typography.bodyLarge,
+            color = MaterialTheme.colorScheme.onSurface,
+            maxLines = 1,
+            overflow = TextOverflow.Ellipsis,
+        )
+        Text(
+            text = summary,
+            style = MaterialTheme.typography.bodySmall,
+            color = MaterialTheme.colorScheme.onSurfaceVariant,
+            maxLines = summaryMaxLines,
+            overflow = TextOverflow.Ellipsis,
+        )
+    }
+    if (time != null) {
+        Text(
+            text = time,
+            // Tabular, because M7 made the headline role tabular
+            // app-wide: a column of times must not jitter as it ticks.
+            style = timeStyle.style,
+            // One line only when capped (seconds on); without seconds
+            // the time wraps at a huge font scale, as it always did.
+            maxLines = if (timeStyle.capped) 1 else Int.MAX_VALUE,
+            softWrap = !timeStyle.capped,
+            overflow = if (timeStyle.capped) TextOverflow.Ellipsis else TextOverflow.Clip,
+            // The row's description already says the time, to the
+            // minute; the ticking text itself stays out of the tree.
+            modifier = Modifier
+                .widthIn(max = timeStyle.maxWidth)
+                .clearAndSetSemantics {},
+            color = timeColor,
+        )
+    }
+}
+
 /**
  * The offset, and the day only when it differs: "Same time as home" stands
  * alone, and "Today" after it would be a sentence saying nothing twice.

app/src/main/java/de/jeanlucmakiola/clockula/ui/worldclock/WorldClockRowState.kt

@@ -17,7 +17,7 @@ import kotlin.time.toJavaInstant
 internal data class WorldClockRowState(
     val id: Long,
     val zoneId: String,
-    /** The user's label when set, else the ICU city, else the id's last segment. Never blank. */
+    /** The user's label when set, else the ICU city, else `ZoneCatalog.cityFallbackFor`. Never blank. */
     val name: String,
     /** Truncated to the minute, or the second with seconds on. Non-null **exactly** when [known]. */
     val time: LocalTime?,
@@ -52,6 +52,13 @@ internal sealed interface WorldClockUiState {
         val canAdd: Boolean,
         /** The user's "show seconds": the readouts' format and the face's second hand. */
         val showSeconds: Boolean,
+        /**
+         * The device's own zone while it is somewhere the time differs from
+         * home (#24); null otherwise. Never stored, never in [rows], so it can
+         * be neither reordered nor removed, and it does not count towards
+         * `WorldClocks.MAX`.
+         */
+        val local: WorldClockRowState? = null,
     ) : WorldClockUiState
 }
 
@@ -93,6 +100,30 @@ internal object WorldClockRows {
         )
     }
 
+    /**
+     * The automatic local clock's row for [deviceZone], read against home like
+     * any other row. Its [WorldClockRowState.id] is [LOCAL_ROW_ID], which no
+     * stored row can have.
+     */
+    fun local(
+        deviceZone: ZoneId,
+        homeZone: ZoneId,
+        now: Instant,
+        entries: Map<String, ZoneEntry>,
+        zoneNames: Map<String, String?>,
+        showSeconds: Boolean,
+    ): WorldClockRowState = from(
+        clocks = listOf(WorldClock(id = LOCAL_ROW_ID, zoneId = deviceZone.id, label = null, sortOrder = 0)),
+        homeZone = homeZone,
+        now = now,
+        entries = entries,
+        zoneNames = zoneNames,
+        showSeconds = showSeconds,
+    ).single()
+
+    /** Room ids start at 1, so a row with this id is never a stored one. */
+    const val LOCAL_ROW_ID: Long = -1L
+
     /** Pure; never throws. */
     fun home(
         homeZone: ZoneId,

app/src/main/java/de/jeanlucmakiola/clockula/ui/worldclock/WorldClockScreen.kt

@@ -134,6 +134,18 @@ fun WorldClockScreen(
                 formatSpokenTime = formatSpokenTime,
             )
 
+            // Above the saved cities and outside their column: it is not one
+            // of them, and a drag must not be able to land between the two.
+            loaded.local?.let { local ->
+                LocalClockRow(
+                    row = local,
+                    formatTime = formatTime,
+                    formatSpokenTime = formatSpokenTime,
+                    timeStyle = rowTimeStyle,
+                )
+                Spacer(Modifier.height(8.dp))
+            }
+
             if (loaded.rows.isEmpty()) {
                 EmptyWorldClocks()
             } else {

app/src/main/java/de/jeanlucmakiola/clockula/ui/worldclock/WorldClockSource.kt

@@ -9,6 +9,7 @@ import de.jeanlucmakiola.clockula.domain.time.Ticker
 import de.jeanlucmakiola.clockula.domain.time.WallClock
 import de.jeanlucmakiola.clockula.domain.time.ZoneProvider
 import de.jeanlucmakiola.clockula.domain.worldclock.HomeZone
+import de.jeanlucmakiola.clockula.domain.worldclock.LocalClock
 import de.jeanlucmakiola.clockula.domain.worldclock.WorldClocks
 import de.jeanlucmakiola.clockula.domain.worldclock.ZoneEntry
 import kotlinx.coroutines.ExperimentalCoroutinesApi
@@ -38,8 +39,12 @@ internal class WorldClockSource @Inject constructor(
     /** What the picker needs before it can draw a row; null while it is still being built. */
     private data class Catalog(val entries: List<ZoneEntry>, val deviceCity: String)
 
+    /** The two settings the tab reads: the stored home zone and the local-clock switch. */
+    private val tabSettings: Flow<Pair<String?, Boolean>> =
+        settings.defaults.map { it.homeZoneId to it.automaticLocalClock }.distinctUntilChanged()
+
     private val storedHomeZoneId: Flow<String?> =
-        settings.defaults.map { it.homeZoneId }.distinctUntilChanged()
+        tabSettings.map { it.first }.distinctUntilChanged()
 
     /**
      * The tab's state. Both the instant and the device zone are re-read on
@@ -50,6 +55,8 @@ internal class WorldClockSource @Inject constructor(
      * registered once per subscription rather than once per list edit, and
      * `onStart` lets the first state through without waiting for a broadcast.
      * `SystemEventReceiver` still needs no new branch (D2).
+     * The same read is what brings the automatic local clock (#24) in and out
+     * as the device's zone changes under an open tab.
      *
      * [refreshes] does the same for what no broadcast announces: the screen
      * sends one on every resume, which covers a per-app language change (the
@@ -66,13 +73,14 @@ internal class WorldClockSource @Inject constructor(
     fun state(refreshes: Flow<Unit>): Flow<WorldClockUiState.Loaded> =
         combine(
             worldClocks.worldClocks(),
-            storedHomeZoneId,
+            tabSettings,
             settings.worldClockSeconds,
             merge(clockChanges.changes(), refreshes).onStart { emit(Unit) },
-        ) { clocks, storedHome, seconds, _ -> Triple(clocks, storedHome, seconds) }
-            .flatMapLatest { (clocks, storedHome, seconds) ->
+        ) { clocks, tab, seconds, _ -> Triple(clocks, tab, seconds) }
+            .flatMapLatest { (clocks, tab, seconds) ->
+                val (storedHome, localClock) = tab
                 ticker.alignedTicks(WorldClockDefaults.tickFor(seconds))
-                    .map { build(clocks, storedHome, seconds) }
+                    .map { build(clocks, storedHome, localClock, seconds) }
             }
             .distinctUntilChanged()
 
@@ -121,11 +129,21 @@ internal class WorldClockSource @Inject constructor(
     private suspend fun build(
         clocks: List<WorldClock>,
         storedHome: String?,
+        localClock: Boolean,
         showSeconds: Boolean,
     ): WorldClockUiState.Loaded {
         val now = wallClock.now()
-        val homeZone = HomeZone.resolve(storedHome, zones.current())
-        val zoneIds = (clocks.map { it.zoneId } + homeZone.id).distinct()
+        val deviceZone = zones.current()
+        val homeZone = HomeZone.resolve(storedHome, deviceZone)
+        val savedZoneIds = clocks.map { it.zoneId }
+        val localZone = LocalClock.zoneToShow(
+            enabled = localClock,
+            homeZone = homeZone,
+            deviceZone = deviceZone,
+            savedZoneIds = savedZoneIds,
+            at = now,
+        )
+        val zoneIds = (savedZoneIds + homeZone.id + listOfNotNull(localZone?.id)).distinct()
         // Both memos live inside the directory, keyed on the locale it reports:
         // a cache here would survive a per-app language change and keep every
         // city in the old language while the ICU zone name switched. Both are
@@ -137,6 +155,7 @@ internal class WorldClockSource @Inject constructor(
             rows = WorldClockRows.from(clocks, homeZone, now, entries, zoneNames, showSeconds),
             canAdd = clocks.size < WorldClocks.MAX,
             showSeconds = showSeconds,
+            local = localZone?.let { WorldClockRows.local(it, homeZone, now, entries, zoneNames, showSeconds) },
         )
     }
 

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

@@ -261,6 +261,8 @@
     <string name="world_clock_limit">Add up to %1$d cities</string>
     <string name="world_clock_home_action">Home time zone</string>
     <string name="world_clock_home">Home</string>
+    <!-- #24: labels the temporary row for the device's own time zone while it differs from home. -->
+    <string name="world_clock_local">Local time</string>
     <string name="world_clock_empty_hint">Add a city to compare its time with your own.</string>
     <string name="world_clock_offset_same">Same time as home</string>
     <string name="world_clock_ahead">%1$s ahead</string>
@@ -350,11 +352,14 @@
     <string name="settings_timer_defaults_hint">How timers ring. A timer can still pick its own sound in its editor.</string>
     <string name="settings_timer_vibrate_off">Off. A timer set to Silent still vibrates</string>
     <string name="settings_section_clock">Clock</string>
-    <string name="settings_clock_subtitle">Home time zone and seconds</string>
+    <string name="settings_clock_subtitle">Home time zone, seconds and local clock</string>
     <string name="settings_clock_hint">World clocks show how far each city is from your home time zone.</string>
     <string name="settings_home_zone">Home time zone</string>
     <string name="settings_world_clock_seconds">Display time with seconds</string>
     <string name="settings_world_clock_seconds_hint">The world clock shows seconds and a second hand. Uses a little more battery.</string>
+    <string name="settings_automatic_local_clock">Automatic local clock</string>
+    <string name="settings_automatic_local_clock_hint">While you’re somewhere the time differs from home, show the local time under the home clock.</string>
+    <string name="settings_automatic_local_clock_needs_home">Needs a home time zone. Home follows the device now, so it’s always local time.</string>
     <string name="settings_self_check">Why might my alarm not ring?</string>
     <string name="settings_self_check_title">Why might my alarm not ring?</string>
     <string name="settings_self_check_summary">Check exact-alarm, notification and battery settings</string>

app/src/test/java/de/jeanlucmakiola/clockula/data/backup/BackupCodecTest.kt

@@ -72,6 +72,7 @@ class BackupCodecTest {
                 alarmSilenceAfterMinutes = 25,
                 timerSilenceAfterMinutes = SilenceAfter.NEVER,
                 volumeButtons = VolumeButtonAction.SNOOZE,
+                automaticLocalClock = false,
             ),
             timerPresets = listOf(1.minutes, 2.minutes, 10.minutes),
             worldClockSeconds = true,
@@ -476,6 +477,17 @@ class BackupCodecTest {
         assertThat(BackupCodec.encode(sampleContent())).contains("\"world_clock_show_seconds\": true")
     }
 
+    @Test
+    fun `a file from before the automatic local clock keeps it on`() {
+        val settings = (BackupCodec.decode(minimalDocument()) as BackupResult.Success).content.settings
+        assertThat(settings.defaults.automaticLocalClock).isTrue()
+    }
+
+    @Test
+    fun `the automatic local clock is written under its on-disk key`() {
+        assertThat(BackupCodec.encode(sampleContent())).contains("\"automatic_local_clock\": false")
+    }
+
     @Test
     fun `an empty preset list is honoured as all removed`() {
         val json = minimalDocument().replace("\"settings\": {}", "\"settings\": {\"timer_presets\": []}")

app/src/test/java/de/jeanlucmakiola/clockula/data/prefs/ClockPrefsTest.kt

@@ -60,6 +60,7 @@ class ClockPrefsTest {
         prefs.setAlarmSilenceAfterMinutes(25)
         prefs.setTimerSilenceAfterMinutes(SilenceAfter.NEVER)
         prefs.setVolumeButtons(VolumeButtonAction.DISMISS)
+        prefs.setAutomaticLocalClock(false)
 
         assertThat(prefs.defaults.first()).isEqualTo(
             ClockDefaults(
@@ -76,6 +77,7 @@ class ClockPrefsTest {
                 alarmSilenceAfterMinutes = 25,
                 timerSilenceAfterMinutes = SilenceAfter.NEVER,
                 volumeButtons = VolumeButtonAction.DISMISS,
+                automaticLocalClock = false,
             ),
         )
     }

app/src/test/java/de/jeanlucmakiola/clockula/data/prefs/SettingsPrefsTest.kt

@@ -91,6 +91,7 @@ class SettingsPrefsTest {
                     volumeButtons = VolumeButtonAction.NOTHING,
                     timerVibrate = false,
                     timerVolumeRampSeconds = 99,
+                    automaticLocalClock = false,
                 ),
                 timerPresets = listOf(5.minutes, 1.minutes, 5.minutes),
                 worldClockSeconds = false,
@@ -107,6 +108,7 @@ class SettingsPrefsTest {
         assertThat(prefs.currentDefaults().volumeButtons).isEqualTo(VolumeButtonAction.NOTHING)
         assertThat(prefs.currentDefaults().timerVibrate).isFalse()
         assertThat(prefs.currentDefaults().timerVolumeRampSeconds).isEqualTo(60)
+        assertThat(prefs.currentDefaults().automaticLocalClock).isFalse()
         assertThat(prefs.timerPresets.first()).containsExactly(1.minutes, 5.minutes).inOrder()
     }
 

app/src/test/java/de/jeanlucmakiola/clockula/domain/worldclock/LocalClockTest.kt

@@ -0,0 +1,98 @@
+package de.jeanlucmakiola.clockula.domain.worldclock
+
+import com.google.common.truth.Truth.assertThat
+import de.jeanlucmakiola.clockula.testing.BERLIN
+import de.jeanlucmakiola.clockula.testing.instantAt
+import org.junit.jupiter.api.Test
+import java.time.ZoneId
+import java.time.ZoneOffset
+import kotlin.time.Instant
+
+/**
+ * #24 — when the device's own zone is shown as a temporary row. "Different"
+ * is a different UTC offset at the instant, as Google Clock decides it.
+ */
+class LocalClockTest {
+
+    private val tokyo: ZoneId = ZoneId.of("Asia/Tokyo")
+    private val paris: ZoneId = ZoneId.of("Europe/Paris")
+    private val london: ZoneId = ZoneId.of("Europe/London")
+    private val july: Instant = instantAt(BERLIN, "2026-07-01T12:00")
+
+    private fun show(
+        storedHome: String?,
+        device: ZoneId,
+        saved: List<String> = emptyList(),
+        enabled: Boolean = true,
+        at: Instant = july,
+    ): ZoneId? = LocalClock.zoneToShow(enabled, HomeZone.resolve(storedHome, device), device, saved, at)
+
+    @Test
+    fun `a device away from a stored home is shown`() {
+        assertThat(show(storedHome = "Europe/Berlin", device = tokyo)).isEqualTo(tokyo)
+    }
+
+    @Test
+    fun `home that follows the device can never differ from it`() {
+        assertThat(show(storedHome = null, device = tokyo)).isNull()
+    }
+
+    @Test
+    fun `a device at home is not shown`() {
+        assertThat(show(storedHome = "Europe/Berlin", device = BERLIN)).isNull()
+    }
+
+    @Test
+    fun `another zone on home's offset is not a different time`() {
+        assertThat(show(storedHome = "Europe/Berlin", device = paris)).isNull()
+    }
+
+    @Test
+    fun `the offset decides, not the zone id`() {
+        // Lisbon and London are different ids on the same offset; Berlin is not.
+        assertThat(show(storedHome = "Europe/Lisbon", device = london)).isNull()
+        assertThat(show(storedHome = "Europe/Berlin", device = london)).isEqualTo(london)
+    }
+
+    @Test
+    fun `a DST change on one side alone can bring it in`() {
+        // Phoenix keeps MST; Denver is on it only in winter.
+        val phoenix = ZoneId.of("America/Phoenix")
+        val january = instantAt(BERLIN, "2026-01-15T12:00")
+        assertThat(show(storedHome = "America/Denver", device = phoenix, at = january)).isNull()
+        assertThat(show(storedHome = "America/Denver", device = phoenix, at = july)).isEqualTo(phoenix)
+    }
+
+    @Test
+    fun `switched off, it is never shown`() {
+        assertThat(show(storedHome = "Europe/Berlin", device = tokyo, enabled = false)).isNull()
+    }
+
+    @Test
+    fun `a saved city on the device's offset already shows the local time`() {
+        assertThat(show(storedHome = "Europe/Berlin", device = tokyo, saved = listOf("Asia/Seoul"))).isNull()
+        assertThat(show(storedHome = "Europe/Berlin", device = tokyo, saved = listOf("Asia/Tokyo"))).isNull()
+    }
+
+    @Test
+    fun `a saved city on another offset does not hide it`() {
+        assertThat(show(storedHome = "Europe/Berlin", device = tokyo, saved = listOf("America/New_York")))
+            .isEqualTo(tokyo)
+    }
+
+    @Test
+    fun `a saved zone the device no longer knows is skipped, not thrown on`() {
+        assertThat(show(storedHome = "Europe/Berlin", device = tokyo, saved = listOf("Mars/Olympus")))
+            .isEqualTo(tokyo)
+    }
+
+    @Test
+    fun `a stored home the device dropped falls back to the device, so nothing differs`() {
+        assertThat(show(storedHome = "Mars/Olympus", device = tokyo)).isNull()
+    }
+
+    @Test
+    fun `a device on a bare offset has no city to show`() {
+        assertThat(show(storedHome = "Europe/Berlin", device = ZoneOffset.ofHours(9))).isNull()
+    }
+}

app/src/test/java/de/jeanlucmakiola/clockula/domain/worldclock/ZoneCatalogTest.kt

@@ -224,4 +224,13 @@ class ZoneCatalogTest {
 
         assertThat(picked).containsExactly("Europe/Kyiv")
     }
+
+    @Test
+    fun `an Etc offset id falls back to its real offset, not its reversed sign`() {
+        assertThat(ZoneCatalog.cityFallbackFor("Etc/GMT-9")).isEqualTo("GMT+9")
+        assertThat(ZoneCatalog.cityFallbackFor("Etc/GMT+5")).isEqualTo("GMT-5")
+        assertThat(ZoneCatalog.cityFallbackFor("Etc/GMT+0")).isEqualTo("GMT")
+        assertThat(ZoneCatalog.cityFallbackFor("Etc/UTC")).isEqualTo("UTC")
+        assertThat(ZoneCatalog.cityFallbackFor("CET")).isEqualTo("CET")
+    }
 }

app/src/test/java/de/jeanlucmakiola/clockula/ui/worldclock/WorldClockRowsTest.kt

@@ -276,4 +276,33 @@ class WorldClockRowsTest {
         assertThat(night.dayFraction).isWithin(0.001f).of(0f)
         assertThat(day.dayFraction).isWithin(0.001f).of(1f)
     }
+
+    // --- local (#24) ---
+
+    @Test
+    fun `the local row is the device's zone, read against home, never stored`() {
+        val local = WorldClockRows.local(ZoneId.of(newYork), BERLIN, january, entries, emptyMap(), showSeconds = false)
+
+        assertThat(local.id).isEqualTo(WorldClockRows.LOCAL_ROW_ID)
+        assertThat(local.name).isEqualTo("New York")
+        assertThat(local.offset).isEqualTo(OffsetLabel.Behind(6, 0))
+        assertThat(local.time).isEqualTo(LocalTime.of(7, 0))
+        assertThat(local.known).isTrue()
+    }
+
+    @Test
+    fun `a device set by UTC offset names the local row by its real offset`() {
+        // Etc/GMT-9 is UTC+9: the id's sign is reversed, so its last segment is no name.
+        val local = WorldClockRows.local(ZoneId.of("Etc/GMT-9"), BERLIN, january, entries, emptyMap(), showSeconds = false)
+
+        assertThat(local.name).isEqualTo("GMT+9")
+        assertThat(local.offset).isEqualTo(OffsetLabel.Ahead(8, 0))
+    }
+
+    @Test
+    fun `a home that follows a device set by UTC offset is named by its real offset`() {
+        val home = WorldClockRows.home(ZoneId.of("Etc/GMT-9"), T0, entries, emptyMap(), showSeconds = false)
+
+        assertThat(home.city).isEqualTo("GMT+9")
+    }
 }

app/src/test/java/de/jeanlucmakiola/clockula/ui/worldclock/WorldClockViewModelTest.kt

@@ -302,6 +302,66 @@ class WorldClockViewModelTest {
         assertThat(harness.loaded().home.zoneId).isEqualTo(harness.zones.current().id)
     }
 
+    /** #24 — the device's zone appears as a temporary row while it is not on home time. */
+    @Test
+    fun `travelling away from a stored home shows a local row`(@TempDir tempDir: Path) =
+        runTest(main.dispatcher) {
+            val harness = collecting(worldClockHarness(tempDir))
+            harness.settings.setHomeZoneId("Europe/Berlin")
+            advanceUntilIdle()
+            assertThat(harness.loaded().local).isNull()
+
+            harness.zones.zone = ZoneId.of("Asia/Tokyo")
+            harness.ticker.tick()
+            advanceUntilIdle()
+
+            val local = harness.loaded().local
+            assertThat(local?.zoneId).isEqualTo("Asia/Tokyo")
+            assertThat(local?.id).isEqualTo(WorldClockRows.LOCAL_ROW_ID)
+            assertThat(harness.loaded().home.zoneId).isEqualTo("Europe/Berlin")
+            assertThat(harness.loaded().rows).isEmpty()
+            assertThat(harness.stored()).isEmpty()
+
+            // Back home, on the next tick, with nothing to clean up.
+            harness.zones.zone = ZoneId.of("Europe/Berlin")
+            harness.ticker.tick()
+            advanceUntilIdle()
+
+            assertThat(harness.loaded().local).isNull()
+        }
+
+    /** #24 */
+    @Test
+    fun `the switch turns the local row off`(@TempDir tempDir: Path) =
+        runTest(main.dispatcher) {
+            val harness = collecting(worldClockHarness(tempDir))
+            harness.zones.zone = ZoneId.of("Asia/Tokyo")
+            harness.settings.setHomeZoneId("Europe/Berlin")
+            advanceUntilIdle()
+            assertThat(harness.loaded().local).isNotNull()
+
+            harness.settings.setAutomaticLocalClock(false)
+            advanceUntilIdle()
+
+            assertThat(harness.loaded().local).isNull()
+        }
+
+    /** #24 — a saved city on the local offset already shows the local time. */
+    @Test
+    fun `a saved city on the device's offset hides the local row`(@TempDir tempDir: Path) =
+        runTest(main.dispatcher) {
+            val dao = FakeWorldClockDao(
+                listOf(WorldClockEntity(id = 1L, zoneId = "Asia/Seoul", label = null, sortOrder = 0)),
+            )
+            val harness = collecting(worldClockHarness(tempDir, dao = dao))
+            harness.zones.zone = ZoneId.of("Asia/Tokyo")
+            harness.settings.setHomeZoneId("Europe/Berlin")
+            advanceUntilIdle()
+
+            assertThat(harness.loaded().local).isNull()
+            assertThat(harness.loaded().rows.map { it.zoneId }).containsExactly("Asia/Seoul")
+        }
+
     /** §5.9 #17 */
     @Test
     fun `the picker's query filters its rows and clearing it restores them`(@TempDir tempDir: Path) =