All timestamps returned via API are in UTC.
Datetimes take the following format:
UNIX timestamps are in seconds, and can contain sub-seconds to microsecond precision after the decimal:
If you see the former format on an endpoint where you would expect the latter format for UNIX timestamps, this just means that the operation occurred exactly on the second boundary, with no sub-second content. So in this case, you would see:
1657922998 and NOT