Fix jump-to-date being off by one day in non-UTC timezones

Closes signalapp/Signal-Android#14816

app/src/main/java/org/thoughtcrime/securesms/conversation/v2/ConversationFragment.kt

@@ -392,6 +392,7 @@ import org.thoughtcrime.securesms.wallpaper.ChatWallpaperDimLevelUtil
 import java.time.Instant
 import java.time.LocalDateTime
 import java.time.ZoneId
+import java.time.ZoneOffset
 import java.util.Locale
 import java.util.Optional
 import java.util.concurrent.ExecutionException
@@ -5317,7 +5318,7 @@ class ConversationFragment :
         datePicker.addOnPositiveButtonClickListener { selectedDate ->
           if (selectedDate != null) {
             val localMidnightTimestamp = Instant.ofEpochMilli(selectedDate)
-              .atZone(ZoneId.systemDefault())
+              .atZone(ZoneOffset.UTC)
               .toLocalDate()
               .atStartOfDay(ZoneId.systemDefault())
               .toInstant()

app/src/main/java/org/thoughtcrime/securesms/conversation/v2/JumpToDateValidator.kt

@@ -16,6 +16,7 @@ import java.time.Instant
 import java.time.LocalDate
 import java.time.LocalDateTime
 import java.time.ZoneId
+import java.time.ZoneOffset
 import java.time.temporal.TemporalAdjusters
 import java.util.concurrent.Executor
 import kotlin.time.Duration.Companion.days
@@ -105,7 +106,7 @@ class JumpToDateValidator private constructor(
 
   private fun normalizeToLocalMidnight(timestamp: Long): Long {
     return Instant.ofEpochMilli(timestamp)
-      .atZone(zoneId)
+      .atZone(ZoneOffset.UTC)
       .toLocalDate()
       .atStartOfDay(zoneId)
       .toInstant()

app/src/test/java/org/thoughtcrime/securesms/conversation/v2/JumpToDateValidatorTest.kt

@@ -187,4 +187,27 @@ class JumpToDateValidatorTest {
 
     assertThat(validator.isValid(june15Utc)).isTrue()
   }
+
+  @Test
+  fun `picker date maps to the same calendar day in a negative-offset zone`() {
+    val newYorkZone = ZoneId.of("America/New_York")
+
+    // Days are keyed by local midnight in the database.
+    val june15NewYorkMidnight = LocalDate.of(2024, 6, 15)
+      .atStartOfDay(newYorkZone)
+      .toInstant()
+      .toEpochMilli()
+
+    val lookup = { dates: Collection<Long> ->
+      dates.associateWith { it == june15NewYorkMidnight }
+    }
+    val validator = createValidator(lookup, zone = newYorkZone)
+
+    // The picker reports June 15 as UTC midnight; it must map to June 15 locally, not June 14.
+    val june15PickerUtc = timestampForDate(2024, 6, 15)
+
+    validator.isValid(june15PickerUtc)
+
+    assertThat(validator.isValid(june15PickerUtc)).isTrue()
+  }
 }