What is Unix time?
Unix time counts the seconds elapsed since 1 January 1970 00:00:00 UTC, ignoring leap seconds. It's the standard time representation in operating systems, databases, log files, and APIs because it's a single unambiguous number with no timezone attached.
Seconds vs milliseconds
Backend systems (Linux, Python's time.time(), most databases) use seconds — a 10-digit number today. JavaScript's Date.now() and many APIs use milliseconds — 13 digits. This converter detects which one you've pasted by length.
Debugging with timestamps
When a log entry or token expiry looks wrong, converting its timestamp is the fastest sanity check: an expiry in the past explains a rejected JWT, and a timestamp of 0 or a 1970 date usually means an uninitialized value in your code.