What is a Unix timestamp?

The number of seconds since 1 January 1970 00:00:00 UTC, not counting leap seconds.

How does it tell seconds from milliseconds?

By size: up to 11 digits is treated as seconds, up to 14 as milliseconds, up to 17 as microseconds, and more as nanoseconds.

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Convert Unix timestamps to dates and dates to timestamps, in any time zone.