After seeing many ads over time saying 'the slower speeds may be off' I thought I should share a simple trick I use for examining shutter speed problems. Download a small and free audio recording program like Audacity or Reaper and use a computer mic to record the shutter. For at least the slow speeds you should be able to zoom in and see how long the shutter opens and closes, with the buzzing in between. I don't know how many other people do this but it works for me. Not super accurate but better than 'eyeballing'/listening for it.