Nikolay Sivov (@nsivov) commented about dlls/mf/tests/mf.c:
- ok(hr == S_OK, "Unexpected hr %#lx.\n", hr);
 - /* Start. */
 - hr = IMFPresentationClock_Start(clock, 0);
 - ok(hr == S_OK, "Unexpected hr %#lx.\n", hr);
 - ret = WaitForSingleObject(stream_event_callback.start_event, 3000);
 - ok(!ret, "WaitForSingleObject returned %#lx.\n", ret);
 - /* Find IDR frame. */
 - while (!is_h264_unit_type(h264_data, H264_UNIT_TYPE_IDR))
 next_h264_unit(&h264_data, &h264_data_size);- /* Process sample. */
 - input_sample = create_h264_sample(h264_data, h264_data_size);
 - hr = IMFSample_SetSampleTime(input_sample, 0);
 - ok(hr == S_OK, "Unexpected hr %#lx.\n", hr);
 
Isn't it zero by default?