fix(cloud_firestore): correct nanoseconds calculation for pre-1970 dates

packages/cloud_firestore/cloud_firestore/example/integration_test/timestamp_e2e.dart

@@ -53,5 +53,22 @@ void runTimestampTests() {
         equals(date.millisecondsSinceEpoch),
       );
     });
+
+    test('set pre-1970 $Timestamp and return', () async {
+      DocumentReference<Map<String, dynamic>> doc =
+          await initializeTest('timestamp');
+      final date = DateTime(1969, 06, 22, 0, 0, 0, 123);
+      final localTimestamp = Timestamp.fromDate(date);
+
+      await doc.set({'foo': localTimestamp});
+
+      DocumentSnapshot<Map<String, dynamic>> snapshot = await doc.get();
+      Timestamp retievedTimestamp = snapshot.data()!['foo'];
+      expect(retievedTimestamp, isA<Timestamp>());
+      expect(
+        retievedTimestamp,
+        equals(localTimestamp),
+      );
+    });
   });
 }

packages/cloud_firestore/cloud_firestore_platform_interface/lib/src/timestamp.dart

@@ -40,8 +40,15 @@ class Timestamp implements Comparable<Timestamp> {
 
   /// Create a [Timestamp] fromMicrosecondsSinceEpoch
   factory Timestamp.fromMicrosecondsSinceEpoch(int microseconds) {
-    final int seconds = microseconds ~/ _kMillion;
-    final int nanoseconds = (microseconds - seconds * _kMillion) * _kThousand;
+    int seconds = microseconds ~/ _kMillion;
+    int nanoseconds = (microseconds - seconds * _kMillion) * _kThousand;
+
+    // Matches implementation in Android SDK:
+    // https://github.com/firebase/firebase-android-sdk/blob/master/firebase-common/src/main/java/com/google/firebase/Timestamp.kt#L114-L121
+    if (nanoseconds < 0) {
+      seconds -= 1;
+      nanoseconds += _kBillion;
+    }
     return Timestamp(seconds, nanoseconds);
   }
 

packages/cloud_firestore/cloud_firestore_platform_interface/test/timestamp_test.dart

@@ -80,5 +80,34 @@ void main() {
 
       expect(epoch, equals(-9999999999));
     });
+
+    test('Timestamp should not throw for dates before 1970', () {
+      final dates = [
+        DateTime(1969, 06, 22, 0, 0, 0, 123),
+        DateTime(1969, 12, 31, 23, 59, 59, 999),
+        DateTime(1900, 01, 01, 12, 30, 45, 500),
+        DateTime(1800, 07, 04, 18, 15, 30, 250),
+        DateTime(0001, 01, 01, 00, 00, 00, 001),
+      ];
+
+      for (final date in dates) {
+        try {
+          final timestamp = Timestamp.fromDate(date);
+          expect(timestamp, isA<Timestamp>());
+        } catch (e) {
+          fail('Timestamp.fromDate threw an error: $e');
+        }
+      }
+    });
+
+    test(
+        'pre-1970 Timestamps should match the original DateTime after conversion',
+        () {
+      final date = DateTime(1969, 06, 22, 0, 0, 0, 123);
+      final timestamp = Timestamp.fromDate(date);
+      final timestampAsDateTime = timestamp.toDate();
+
+      expect(date, equals(timestampAsDateTime));
+    });
   });
 }