diff --git a/internal/sync/sync.go b/internal/sync/sync.go
index 912c1ec34..a84a9982c 100644
--- a/internal/sync/sync.go
+++ b/internal/sync/sync.go
@@ -216,7 +216,11 @@ func (s *Syncer) processBatch(ctx context.Context, sourceID int64, listResp *gma
 			// Track oldest message date for progress display
 			// Gmail returns messages newest-to-oldest, so oldest shows where we've reached
 			if raw.InternalDate > 0 {
-				msgDate := time.UnixMilli(raw.InternalDate)
+				// .UTC() to match how InternalDate is normalized everywhere
+				// else (the stored message date and parsed.Date both use
+				// .UTC()); without it oldestDate carries the local zone and
+				// callers reading its calendar day are off by one east of UTC.
+				msgDate := time.UnixMilli(raw.InternalDate).UTC()
 				if result.oldestDate.IsZero() || msgDate.Before(result.oldestDate) {
 					result.oldestDate = msgDate
 				}
diff --git a/internal/whatsapp/importer.go b/internal/whatsapp/importer.go
index f37a28389..8cbf28897 100644
--- a/internal/whatsapp/importer.go
+++ b/internal/whatsapp/importer.go
@@ -468,7 +468,7 @@ func (imp *Importer) Import(ctx context.Context, waDBPath string, opts ImportOpt
 							reactorID = selfParticipantID
 						}
 
-						createdAt := time.Unix(r.Timestamp/1000, 0)
+						createdAt := time.Unix(r.Timestamp/1000, 0).UTC()
 						if err := imp.store.UpsertReaction(messageID, reactorID, reactionTypeEmoji, reactionValue, createdAt); err != nil {
 							summary.Errors++
 							imp.progress.OnError(fmt.Errorf("upsert reaction: %w", err))
diff --git a/internal/whatsapp/mapping.go b/internal/whatsapp/mapping.go
index ad4d9d51f..023158b76 100644
--- a/internal/whatsapp/mapping.go
+++ b/internal/whatsapp/mapping.go
@@ -42,8 +42,12 @@ func mapMessage(msg waMessage, conversationID, sourceID int64, senderID sql.Null
 	sentAt := sql.NullTime{}
 	if msg.Timestamp > 0 {
 		// WhatsApp timestamps are in milliseconds since epoch.
+		// .UTC() to match how every other importer normalizes stored
+		// message dates; without it SentAt carries the local zone and
+		// Parquet year-partitioning / calendar-day reads are off by one
+		// near boundaries east of UTC.
 		sentAt = sql.NullTime{
-			Time:  time.Unix(msg.Timestamp/1000, (msg.Timestamp%1000)*1e6),
+			Time:  time.Unix(msg.Timestamp/1000, (msg.Timestamp%1000)*1e6).UTC(),
 			Valid: true,
 		}
 	}
diff --git a/internal/whatsapp/mapping_test.go b/internal/whatsapp/mapping_test.go
index 91e58852e..e1315ae11 100644
--- a/internal/whatsapp/mapping_test.go
+++ b/internal/whatsapp/mapping_test.go
@@ -284,3 +284,20 @@ func TestChatTitle(t *testing.T) {
 	}
 	assertpkg.Equal(t, "+447700900000", chatTitle(direct), "chatTitle(direct)")
 }
+
+func TestMapMessageSentAtIsUTC(t *testing.T) {
+	assert := assertpkg.New(t)
+
+	// 2024-01-10T12:00:00Z in milliseconds. On a machine east of UTC
+	// (e.g. NZ) a non-UTC SentAt would read back as Jan 11 and carry the
+	// local zone — wrong for Parquet year-partitioning and date display.
+	msg := waMessage{Timestamp: 1704888000000}
+	got := mapMessage(msg, 1, 1, sql.NullInt64{})
+
+	requirepkg.True(t, got.SentAt.Valid, "SentAt should be set")
+	assert.Equal("UTC", got.SentAt.Time.Location().String(), "SentAt must be normalized to UTC")
+	y, m, d := got.SentAt.Time.Date()
+	assert.Equal(2024, y, "year")
+	assert.Equal(1, int(m), "month")
+	assert.Equal(10, d, "day")
+}