What is a Unix timestamp?
A Unix timestamp is the number of seconds since 00:00:00 UTC on January 1, 1970 (the Unix epoch), not counting leap seconds. It is the same everywhere in the world, which makes it ideal for storing times in databases and APIs.
Seconds or milliseconds?
Most systems use seconds (10 digits today), while JavaScript and many APIs use milliseconds (13 digits). The converter treats values of 12 or more digits as milliseconds.
The year 2038 problem
Systems that store the timestamp as a signed 32-bit integer overflow after 2,147,483,647, which is 03:14:07 UTC on January 19, 2038. Modern systems use 64-bit values.