What is a Unix timestamp?
A Unix timestamp counts elapsed seconds since 1970-01-01T00:00:00Z. It represents an instant in time; time zones only change how that instant is displayed.
Seconds, milliseconds and ISO 8601
Most Unix timestamps use seconds. JavaScript often uses milliseconds. ISO 8601 is a readable standard such as 2026-09-28T12:34:56.000Z, where Z means UTC.