diff --git a/internal/store/time.go b/internal/store/time.go
index 00be284..05a0401 100644
--- a/internal/store/time.go
+++ b/internal/store/time.go
@@ -47,6 +47,9 @@ func (t *Timestamp) Scan(src any) error {
 	if err != nil {
 		return fmt.Errorf("store: scan Timestamp %q: %w", s, err)
 	}
+	if parsed.Format(timestampLayout) != s {
+		return fmt.Errorf("store: scan Timestamp %q: want %s", s, timestampLayout)
+	}
 	t.Time = parsed
 	return nil
 }
diff --git a/internal/store/time_test.go b/internal/store/time_test.go
index 5c77225..eb7d1cc 100644
--- a/internal/store/time_test.go
+++ b/internal/store/time_test.go
@@ -60,7 +60,10 @@ func TestTimestampRejects(t *testing.T) {
 	if _, err := (Timestamp{}).Value(); err == nil {
 		t.Error("zero Timestamp Value succeeded")
 	}
-	for _, src := range []any{nil, int64(0), []byte("2026-10-03T12:37:05.123Z"), "2026-10-03T12:37:05Z", "2026-10-03T08:37:05.123-04:00", "garbage"} {
+	for _, src := range []any{
+		nil, int64(0), []byte("2026-10-03T12:37:05.123Z"), "2026-10-03T12:37:05Z",
+		"2026-10-03T08:37:05.123-04:00", "2026-10-03T12:37:05,123Z", "garbage",
+	} {
 		var ts Timestamp
 		if err := ts.Scan(src); err == nil {
 			t.Errorf("Scan(%#v) succeeded", src)