millisecondsSinceEpoch is acting like is dependent to timezone in dart

I have found some weird issues while running tests in our CI. The tests are flaky even tho we are mocking our function to return a fixed date. Running the same code returns a different value for millisecondsSinceEpoch

Here you can see the bit of code that causes the issue:

String fileName = "${event.gymName.replaceAll(RegExp(r'\s'), "-")}-${dateUtil.getCurrentDateTime().millisecondsSinceEpoch}.png";
String gymImage = await storageRepository.uploadImage(croppedFile, fileName);

and this is the test that fail in the CI but not in my local (only relevant bit):

group("on UploadGymImage event", () {
      final testDate = DateTime(2021, 1, 11);
      var newImageUrl = "https://gym.img/abc.png";

      setUp(() {
        when(mockStorageRepository.uploadImage(fakeImage, argThat(endsWith(".png"))))
            .thenAnswer((_) => Future.value(newImageUrl));
        when(mockDateUtil.getCurrentDateTime()).thenReturn(testDate);
      });

      tearDown(() async {
        verify(mockStorageRepository.uploadImage(
          fakeImage,
          "a-gym-1610319600000.png",
        ));
        verify(mockDateUtil.getCurrentDateTime());
      });
    });

The documentation for millisecondsSinceEpoch says:

  /// The number of milliseconds since
  /// the "Unix epoch" 1970-01-01T00:00:00Z (UTC).
  ///
  /// This value is independent of the time zone.
  ///
  /// This value is at most
  /// 8,640,000,000,000,000ms (100,000,000 days) from the Unix epoch.
  /// In other words: `millisecondsSinceEpoch.abs() <= 8640000000000000`.

So I thought that by returning a fixed date in my mock, there would not be an issue running this test anywhere.

Can someone explain to me why is so?

note: the CI returns 1610323200000 my local instead 1610319600000