What's the issue?

AMP runtime should use performance.now() instead of Date.now() when recording timestamps for tick events. Clock changes such as those triggered by software like NTP, crossing daylight savings boundaries, crossing leap days / seconds, etc all can affect Date.now making it unreliable for precision timing.

I realize that the viewer's performance clock and the runtime's performance clock can be different. Maybe returning performance.now() + performance.timing.navigationStart would maintain the same interface but has the monotonic property that Date.now() lacks.

What browsers are affected?

All browsers

Which AMP version is affected?

All AMP versions afaik