Unix timestamp is the number of seconds since January 1, 1970, 00:00:00 UTC (the Unix epoch).

JavaScript uses milliseconds, so multiply by 1000 when converting to a Date object.

Negative timestamps represent dates before the Unix epoch (January 1, 1970). For example, -86400 is December 31, 1969.

This tool auto-detects milliseconds vs seconds based on magnitude. Values with absolute value greater than 9,999,999,999 are treated as milliseconds.