Clarify the difference between seconds, milliseconds, and timezone display so users make fewer timestamp mistakes.
Identify the unit before worrying about the format
Ten digits usually means seconds and thirteen digits usually means milliseconds. That is the quick rule of thumb.
In practice, the safer approach is to let people switch between both units and see the result immediately.
Why local time and UTC should both be shown
A lot of “the time looks wrong” issues come from display timezone differences, not from the timestamp itself.
Showing both local time and UTC helps people confirm whether the problem is the unit or the timezone much faster.