What is a Unix timestamp?
Unix time (epoch time) counts the seconds since 1 January 1970, 00:00:00 UTC, ignoring leap seconds. It's how most computers, databases and APIs store time, because it's a single number that's the same in every time zone.
Seconds or milliseconds?
Unix and most back-end systems use seconds (10 digits today). JavaScript's Date.now() and many APIs use milliseconds (13 digits). This converter detects which one you entered automatically.
The year 2038 problem
Systems that store Unix time as a signed 32-bit number overflow on 19 January 2038 at 03:14:07 UTC (timestamp 2147483647). Modern 64-bit systems are not affected.