feat: add Timestamp.zero and Timestamp.difference()

firestore/lib/src/timestamp.dart

@@ -205,6 +205,9 @@ class Timestamp implements Comparable<Timestamp?> {
       return tryParse(any.toString());
     }
   }
+
+  /// The minimum representable [Timestamp]
+  static final zero = Timestamp(0, 0);
 }
 
 const _nanosPerSeconds = 1000000000;
@@ -234,4 +237,22 @@ extension TekartikFirestoreTimestampExt on Timestamp {
   Timestamp substractDuration(Duration duration) {
     return _addMicroseconds(-duration.inMicroseconds);
   }
+
+  /// Returns a [Duration] with the difference when subtracting [other] from
+  /// this [DateTime].
+  ///
+  /// The returned [Duration] will be negative if [other] occurs after this
+  /// [DateTime].
+  ///
+  ///
+  /// The difference is measured in seconds and fractions of seconds.
+  Duration difference(Timestamp other) {
+    var diffSeconds = seconds - other.seconds;
+    var diffNanos = nanoseconds - other.nanoseconds;
+    if (diffNanos < 0) {
+      diffSeconds--;
+      diffNanos += _nanosPerSeconds;
+    }
+    return Duration(seconds: diffSeconds, microseconds: diffNanos ~/ 1000);
+  }
 }

firestore/test/timestamp_test.dart

@@ -224,5 +224,23 @@ void main() {
               .substractDuration(const Duration(seconds: 2, microseconds: 400)),
           Timestamp(2, 999900000));
     });
+    test('difference', () {
+      expect(Timestamp(3, 1000).difference(Timestamp(3, 2000)),
+          const Duration(microseconds: -1));
+      expect(Timestamp(3, 2000).difference(Timestamp(3, 1000)),
+          const Duration(microseconds: 1));
+      expect(Timestamp(2, 1000).difference(Timestamp(3, 2000)),
+          const Duration(microseconds: -1000001));
+      expect(Timestamp(3, 1000).difference(Timestamp(2, 2000)),
+          const Duration(microseconds: 999999));
+      expect(Timestamp(62, 1000).difference(Timestamp(1, 2000)),
+          const Duration(minutes: 1, microseconds: 999999));
+      var now = DateTime.timestamp();
+      expect(Timestamp.fromDateTime(now).difference(Timestamp.zero),
+          now.difference(Timestamp.zero.toDateTime(isUtc: true)));
+    });
+    test('zero', () {
+      expect(Timestamp.zero.toIso8601String(), '1970-01-01T00:00:00.000Z');
+    });
   });
 }