A Unix timestamp counts seconds since 1 January 1970 (UTC). Paste one to read it, or pick a date to get its timestamp. Millisecond timestamps are detected automatically.
Questions
Seconds or milliseconds?
Numbers longer than 11 digits are treated as milliseconds (JavaScript style), shorter ones as seconds.
What is the year 2038 problem?
Systems storing timestamps as 32-bit signed integers overflow on 19 January 2038. 64-bit timestamps don’t have this problem.