Unix time (also called epoch time or POSIX time) is the number of seconds that have elapsed since January 1, 1970 UTC. It's the closest thing computer science has to a universal, unambiguous clock — which is why it powers everything from your database timestamps to JWT expiry claims.
Seconds or milliseconds?
Here lies the eternal footgun. Most Unix-y systems (Postgres, Python time.time(), curl, JWT exp) use seconds since epoch, while JavaScript's Date.now() uses milliseconds. A 10-digit number is almost certainly seconds; a 13-digit number is almost certainly milliseconds. Our converter auto-detects this.
Timezones don't exist in Unix time
The entire point of Unix time is to be timezone-neutral. It represents a single instant on the global timeline. Timezones only enter the picture when you format that instant for human display. 1717171717 is the same moment whether you're in Tokyo or Toronto — only the wall-clock display differs.
The year 2038 problem
32-bit signed integers overflow at 03:14:07 UTC on 19 January 2038. Modern systems have almost all migrated to 64-bit which pushes the ceiling to roughly 292 billion years in the future — further than the estimated lifetime of stars.
Practical conversion tips
- To convert a Unix timestamp in JavaScript:
new Date(1717171717 * 1000)(multiply by 1000 for seconds → milliseconds). - In SQL:
to_timestamp(1717171717)in Postgres, orFROM_UNIXTIME(1717171717)in MySQL. - Store timestamps in your database as
timestamptzorbigint, never as strings. - When displaying to users, always convert to their local timezone at the last possible moment.
Bookmark the Unix Timestamp Converter — you'll use it more often than you'd think.