See also the discussion of macos timing on Discussion: Frame timing on MacOS
It looks to me like Apple have done some new funky thing (since we ran the mega timing study) in the OS to make graphics smoother for most users, but less accurate for us. I would say on MacOS you can no longer get stable frame timing and I would not recommend it for any short duration stimuli 
For longer duration stimuli where you probably don’t care about a single screen refresh being added or lost, then set the durations to be time-based on MacOS
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