feat: daily question date key uses local timezone instead of UTC (batch v0.2.15)

- Replace SimpleDateFormat/Calendar UTC date key with java.time LocalDate in device timezone
- FirestoreAnswerDataSource: todayLocalDateString() with injectable Clock, localDateString() helper
- DailyQuestionViewModel: pass date through submit flow so sync uses same date key
- AnswerRevealViewModel: use todayLocalDateString() for partner answer lookup
- Add FirestoreAnswerDataSourceTest: verifies timezone-aware date boundaries (Chicago vs Tokyo, LA vs London)

app/src/main/java/app/closer/data/remote/FirestoreAnswerDataSource.kt

@@ -6,10 +6,11 @@ import app.closer.domain.model.LocalAnswer
 import com.google.firebase.firestore.FirebaseFirestore
 import kotlinx.coroutines.suspendCancellableCoroutine
 import org.json.JSONArray
-import java.text.SimpleDateFormat
-import java.util.Calendar
-import java.util.Locale
-import java.util.TimeZone
+import java.time.Clock
+import java.time.Instant
+import java.time.LocalDate
+import java.time.ZoneId
+import java.time.format.DateTimeFormatter
 import javax.inject.Inject
 import javax.inject.Singleton
 import kotlin.coroutines.resume
@@ -47,9 +48,9 @@ class FirestoreAnswerDataSource @Inject constructor(
         coupleId: String,
         questionId: String,
         userId: String,
-        answer: LocalAnswer
+        answer: LocalAnswer,
+        date: String = todayLocalDateString()
     ): Unit = suspendCancellableCoroutine { cont ->
-        val date = todayUtcString()
         val aead = encryptionManager.requireAead(coupleId)
         val data = mapOf(
             "userId" to userId,
@@ -74,12 +75,12 @@ class FirestoreAnswerDataSource @Inject constructor(
     }
 
     /**
-     * Fetches a partner's answer for the current UTC date.
+     * Fetches a partner's answer for the current local date.
      */
     suspend fun getAnswerForUser(
         coupleId: String,
         userId: String,
-        date: String = todayUtcString()
+        date: String = todayLocalDateString()
     ): LocalAnswer? = suspendCancellableCoroutine { cont ->
         answerRef(coupleId, date, userId)
             .get()
@@ -97,7 +98,7 @@ class FirestoreAnswerDataSource @Inject constructor(
     /**
      * Reads the couple-scoped daily question assignment for today.
      */
-    suspend fun getDailyQuestionAssignment(coupleId: String, date: String = todayUtcString()): DailyQuestionAssignment? =
+    suspend fun getDailyQuestionAssignment(coupleId: String, date: String = todayLocalDateString()): DailyQuestionAssignment? =
         suspendCancellableCoroutine { cont ->
             dailyQuestionRef(coupleId, date)
                 .get()
@@ -122,7 +123,7 @@ class FirestoreAnswerDataSource @Inject constructor(
      * Calls the cloud function to assign a daily question for the couple immediately.
      * Used when a couple is newly created and has no assignment yet.
      */
-    suspend fun requestDailyQuestionAssignment(coupleId: String, date: String = todayUtcString()): Unit =
+    suspend fun requestDailyQuestionAssignment(coupleId: String, date: String = todayLocalDateString()): Unit =
         suspendCancellableCoroutine { cont ->
             val functions = com.google.firebase.functions.FirebaseFunctions.getInstance()
             functions
@@ -180,12 +181,20 @@ class FirestoreAnswerDataSource @Inject constructor(
     )
 
     companion object {
-        // UTC keeps both partners on the same date key regardless of where they are.
-        // The Cloud Functions assignDailyQuestion must also use UTC when creating date docs.
-        fun todayUtcString(): String {
-            val fmt = SimpleDateFormat("yyyy-MM-dd", Locale.US)
-            fmt.timeZone = TimeZone.getTimeZone("UTC")
-            return fmt.format(Calendar.getInstance(TimeZone.getTimeZone("UTC")).time)
+        private val dateKeyFormatter: DateTimeFormatter = DateTimeFormatter.ISO_LOCAL_DATE
+
+        /**
+         * Daily-question document IDs follow the user's local calendar day.
+         *
+         * Firestore timestamps remain absolute instants, but the `daily_question/{date}`
+         * key should match what the user experiences as "today" on their device.
+         */
+        fun todayLocalDateString(clock: Clock = Clock.systemDefaultZone()): String {
+            return LocalDate.now(clock).format(dateKeyFormatter)
+        }
+
+        fun localDateString(instant: Instant, zoneId: ZoneId = ZoneId.systemDefault()): String {
+            return instant.atZone(zoneId).toLocalDate().format(dateKeyFormatter)
         }
     }
 }

app/src/main/java/app/closer/ui/answers/AnswerRevealViewModel.kt

@@ -62,7 +62,7 @@ class AnswerRevealViewModel @Inject constructor(
                         firestoreAnswerDataSource.getAnswerForUser(
                             coupleId = coupleId,
                             userId = partnerId,
-                            date = FirestoreAnswerDataSource.todayUtcString()
+                            date = FirestoreAnswerDataSource.todayLocalDateString()
                         )
                     }.onFailure { crashReporter.recordException(it) }.getOrNull()
                 } else null
@@ -115,7 +115,7 @@ class AnswerRevealViewModel @Inject constructor(
                 firestoreAnswerDataSource.getAnswerForUser(
                     coupleId = coupleId,
                     userId = partnerId,
-                    date = FirestoreAnswerDataSource.todayUtcString()
+                    date = FirestoreAnswerDataSource.todayLocalDateString()
                 )
             }.onFailure { crashReporter.recordException(it) }.getOrNull()
             partnerAnswer?.let { _uiState.update { it.copy(partnerAnswer = partnerAnswer) } }

app/src/main/java/app/closer/ui/questions/DailyQuestionViewModel.kt

@@ -24,6 +24,7 @@ data class LocalQuestionUiState(
     val error: String? = null,
     val question: Question? = null,
     val coupleId: String? = null,
+    val dailyQuestionDate: String? = null,
     val submitted: Boolean = false,
     val pendingWrittenText: String = "",
     val pendingSelectedOptionIds: List<String> = emptyList(),
@@ -51,12 +52,14 @@ class DailyQuestionViewModel @Inject constructor(
         viewModelScope.launch {
             _uiState.value = LocalQuestionUiState(isLoading = true)
             try {
-                val (coupleId, question) = loadCoupleAndQuestion()
+                val today = FirestoreAnswerDataSource.todayLocalDateString()
+                val (coupleId, question) = loadCoupleAndQuestion(today)
                 val answer = question?.let { localAnswerRepository.getAnswer(it.id) }
                 _uiState.value = LocalQuestionUiState(
                     isLoading = false,
                     question = question,
                     coupleId = coupleId,
+                    dailyQuestionDate = today,
                     pendingScaleValue = defaultScaleValue(question)
                 ).withLocalAnswer(answer)
             } catch (e: Exception) {
@@ -79,7 +82,7 @@ class DailyQuestionViewModel @Inject constructor(
      *
      * For unpaired users, fall back to the local random question pool.
      */
-    private suspend fun loadCoupleAndQuestion(): Pair<String?, Question?> {
+    private suspend fun loadCoupleAndQuestion(today: String): Pair<String?, Question?> {
         val couple = authRepository.currentUserId?.let { uid ->
             runCatching { coupleRepository.getCoupleForUser(uid) }
                 .onFailure { crashReporter.recordException(it) }
@@ -91,7 +94,6 @@ class DailyQuestionViewModel @Inject constructor(
         }
 
         val coupleId = couple.id
-        val today = FirestoreAnswerDataSource.todayUtcString()
         val assignment = runCatching {
             firestoreAnswerDataSource.getDailyQuestionAssignment(coupleId, today)
         }.onFailure { crashReporter.recordException(it) }.getOrNull()
@@ -146,7 +148,7 @@ class DailyQuestionViewModel @Inject constructor(
             val localAnswer = state.toLocalAnswer(question).copy(updatedAt = System.currentTimeMillis())
             localAnswerRepository.saveAnswer(localAnswer)
             _uiState.update { it.copy(submitted = true) }
-            syncAnswerToFirestore(state.coupleId, question.id, localAnswer)
+            syncAnswerToFirestore(state.coupleId, state.dailyQuestionDate, question.id, localAnswer)
         }
     }
 
@@ -170,11 +172,17 @@ class DailyQuestionViewModel @Inject constructor(
      * Syncs the local answer to Firestore so the partner can see it. Failures are
      * non-blocking: the user's answer is already persisted locally.
      */
-    private suspend fun syncAnswerToFirestore(coupleId: String?, questionId: String, answer: LocalAnswer) {
+    private suspend fun syncAnswerToFirestore(
+        coupleId: String?,
+        dailyQuestionDate: String?,
+        questionId: String,
+        answer: LocalAnswer
+    ) {
         val userId = authRepository.currentUserId ?: return
         if (coupleId.isNullOrBlank()) return
+        if (dailyQuestionDate.isNullOrBlank()) return
         runCatching {
-            firestoreAnswerDataSource.saveAnswer(coupleId, questionId, userId, answer)
+            firestoreAnswerDataSource.saveAnswer(coupleId, questionId, userId, answer, dailyQuestionDate)
         }.onFailure {
             crashReporter.recordException(it)
         }

app/src/test/java/app/closer/data/remote/FirestoreAnswerDataSourceTest.kt

@@ -0,0 +1,35 @@
+package app.closer.data.remote
+
+import java.time.Clock
+import java.time.Instant
+import java.time.ZoneId
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class FirestoreAnswerDataSourceTest {
+
+    @Test
+    fun `today local date string uses the clock timezone`() {
+        val instant = Instant.parse("2026-06-20T02:30:00Z")
+
+        val chicagoClock = Clock.fixed(instant, ZoneId.of("America/Chicago"))
+        val tokyoClock = Clock.fixed(instant, ZoneId.of("Asia/Tokyo"))
+
+        assertEquals("2026-06-19", FirestoreAnswerDataSource.todayLocalDateString(chicagoClock))
+        assertEquals("2026-06-20", FirestoreAnswerDataSource.todayLocalDateString(tokyoClock))
+    }
+
+    @Test
+    fun `local date string converts a UTC instant into the requested local zone`() {
+        val instant = Instant.parse("2026-01-01T00:30:00Z")
+
+        assertEquals(
+            "2025-12-31",
+            FirestoreAnswerDataSource.localDateString(instant, ZoneId.of("America/Los_Angeles"))
+        )
+        assertEquals(
+            "2026-01-01",
+            FirestoreAnswerDataSource.localDateString(instant, ZoneId.of("Europe/London"))
+        )
+    }
+}