Unix timestamp (or Epoch time) is the number of seconds that have elapsed since January 1, 1970 00:00:00 UTC. Its widely used in programming for storing and comparing dates.

32-bit systems store timestamps as signed integers, max value 2,147,483,647 (Jan 19, 2038 03:14:07 UTC). After this, the value overflows. Modern systems use 64-bit integers to avoid this.

Unix timestamps can be in seconds (10 digits) or milliseconds (13 digits). JavaScript Date.now() returns milliseconds. Many APIs use seconds. Check the digit count to determine the format.