fix(sleep): prevent epoch dates on Sleep startup (#137)

app/src/main/java/com/pulseloop/ui/screens/SleepScreen.kt

@@ -874,6 +874,7 @@ private fun SleepDurationHistogram(
 @Composable
 private fun SleepDayNavHeader(state: SleepViewModel.SleepState, viewModel: SleepViewModel?) {
     var showPicker by remember { mutableStateOf(false) }
+    val shownDayMillis = resolvedSleepDayMillis(state.shownDayMillis)
     val canOlder = state.dayOffset < state.maxDayOffset
     val canNewer = state.dayOffset > 0
 
@@ -892,10 +893,10 @@ private fun SleepDayNavHeader(state: SleepViewModel.SleepState, viewModel: Sleep
             horizontalAlignment = Alignment.CenterHorizontally,
         ) {
             Text(
-                dayLabel(state.shownDayMillis),
+                dayLabel(shownDayMillis),
                 fontSize = 16.sp, fontWeight = FontWeight.SemiBold, color = PulseColors.textPrimary,
             )
-            val sub = daySubLabel(state.shownDayMillis)
+            val sub = daySubLabel(shownDayMillis)
             Text(
                 sub.ifEmpty { " " },
                 fontSize = 11.sp, fontWeight = FontWeight.Medium,
@@ -907,14 +908,21 @@ private fun SleepDayNavHeader(state: SleepViewModel.SleepState, viewModel: Sleep
 
     if (showPicker) {
         SleepDayPickerDialog(
-            shownDayMillis = state.shownDayMillis,
+            shownDayMillis = shownDayMillis,
             maxDayOffset = state.maxDayOffset,
             onDismiss = { showPicker = false },
             onPick = { viewModel?.jumpToDay(it); showPicker = false },
         )
     }
 }
 
+/** Missing day keys must never reach the header or date picker as Unix-epoch dates. */
+internal fun resolvedSleepDayMillis(
+    dayMillis: Long,
+    nowMs: Long = System.currentTimeMillis(),
+    zone: java.time.ZoneId = java.time.ZoneId.systemDefault(),
+): Long = if (dayMillis > 0L) dayMillis else com.pulseloop.util.TimeUtil.referenceNightLocal(nowMs, zone)
+
 @Composable
 private fun NavChevron(
     icon: androidx.compose.ui.graphics.vector.ImageVector,

app/src/main/java/com/pulseloop/ui/viewmodels/ViewModels.kt

@@ -224,8 +224,8 @@ class SleepViewModel(
         val dayOffset: Int = 0,
         /** How far back the stepper may page (0 locks to today when there's no history). */
         val maxDayOffset: Int = 0,
-        /** Local-midnight key of the shown day, for the nav header label. */
-        val shownDayMillis: Long = 0L,
+        /** Local-midnight key of the shown day; valid even before the first database emission. */
+        val shownDayMillis: Long = TimeUtil.referenceNightLocal(),
         val recentSessions: List<SleepSessionEntity> = emptyList(),
         // Aggregate view (week/month/year)
         val rangeSessions: List<SleepSessionEntity> = emptyList(),

app/src/test/java/com/pulseloop/ui/screens/SleepDayDateTest.kt

@@ -0,0 +1,79 @@
+package com.pulseloop.ui.screens
+
+import com.pulseloop.ui.viewmodels.SleepViewModel
+import com.pulseloop.util.TimeUtil
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import java.time.LocalDate
+import java.time.LocalDateTime
+import java.time.ZoneId
+import java.util.TimeZone
+
+class SleepDayDateTest {
+    private val zones = listOf("America/New_York", "UTC", "Asia/Tokyo").map(ZoneId::of)
+
+    private fun at(dateTime: String, zone: ZoneId): Long =
+        LocalDateTime.parse(dateTime).atZone(zone).toInstant().toEpochMilli()
+
+    private fun midnight(date: String, zone: ZoneId): Long =
+        LocalDate.parse(date).atStartOfDay(zone).toInstant().toEpochMilli()
+
+    @Test
+    fun `initial empty sleep state has a reference date without database or ring data`() {
+        val originalZone = TimeZone.getDefault()
+        try {
+            for (zone in zones) {
+                TimeZone.setDefault(TimeZone.getTimeZone(zone))
+                val before = TimeUtil.referenceNightLocal()
+                val state = SleepViewModel.SleepState()
+                val after = TimeUtil.referenceNightLocal()
+
+                assertTrue("Initial date in $zone", state.shownDayMillis in setOf(before, after))
+                assertTrue(state.daySessions.isEmpty())
+                assertEquals(state.shownDayMillis, resolvedSleepDayMillis(state.shownDayMillis))
+            }
+        } finally {
+            TimeZone.setDefault(originalZone)
+        }
+    }
+
+    @Test
+    fun `zero and negative dates resolve to today from 4am in every zone`() {
+        for (zone in zones) {
+            val now = at("2026-11-04T04:00:00", zone)
+            for (invalid in listOf(0L, -1L, Long.MIN_VALUE)) {
+                assertEquals("Invalid $invalid in $zone", midnight("2026-11-04", zone),
+                    resolvedSleepDayMillis(invalid, now, zone))
+            }
+        }
+    }
+
+    @Test
+    fun `missing dates before 4am resolve to the previous reference night`() {
+        for (zone in zones) {
+            val now = at("2026-11-04T03:59:59", zone)
+            assertEquals(midnight("2026-11-03", zone), resolvedSleepDayMillis(0L, now, zone))
+        }
+    }
+
+    @Test
+    fun `fallback respects local midnight across daylight saving transitions`() {
+        val zone = ZoneId.of("America/New_York")
+        for ((now, expectedDay) in listOf(
+            "2026-03-09T01:30:00" to "2026-03-08",
+            "2026-11-02T01:30:00" to "2026-11-01",
+        )) {
+            assertEquals(midnight(expectedDay, zone), resolvedSleepDayMillis(0L, at(now, zone), zone))
+        }
+    }
+
+    @Test
+    fun `valid historical selection is preserved for header and picker`() {
+        for (zone in zones) {
+            val selected = midnight("2026-10-25", zone)
+            val now = at("2026-11-04T12:00:00", zone)
+            assertEquals(selected, resolvedSleepDayMillis(selected, now, zone))
+        }
+    }
+}