Serialize Timestamps as string (per proto3 JSON format).

Author: yuchenshi

packages/firestore/src/remote/serializer.ts

@@ -207,11 +207,15 @@ export class JsonProtoSerializer {
    * to actually return a Timestamp proto.
    */
   private toTimestamp(timestamp: Timestamp): string {
-    return {
-      seconds: '' + timestamp.seconds,
-      nanos: timestamp.nanoseconds
-      // eslint-disable-next-line @typescript-eslint/no-explicit-any
-    } as any;
+    if (this.options.useProto3Json) {
+      return timestamp.toDate().toISOString();
+    } else {
+      return {
+        seconds: '' + timestamp.seconds,
+        nanos: timestamp.nanoseconds
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      } as any;
+    }
   }
 
   private fromTimestamp(date: string | TimestampProto): Timestamp {