What is Unix Timestamp?
The number of seconds elapsed since January 1, 1970 00:00:00 UTC.
A Unix timestamp (also called Epoch time or POSIX time) counts the number of seconds that have elapsed since the Unix Epoch: January 1, 1970, 00:00:00 UTC. It is the universal standard for representing dates and times in computing systems.
Unix timestamps are timezone-agnostic — the same timestamp represents the same instant in time everywhere on Earth. Converting to a human-readable date requires knowing the target timezone.
Seconds vs Milliseconds
Unix timestamps are traditionally in seconds. JavaScript's Date.now() returns milliseconds. A 10-digit timestamp is seconds; a 13-digit timestamp is milliseconds. This is a common source of bugs.
The Year 2038 Problem
32-bit systems store Unix timestamps as signed integers. On January 19, 2038 at 03:14:07 UTC, the value will overflow. Most modern systems use 64-bit timestamps which are safe for billions of years.