Convert a Unix timestamp into a readable date, or a date back into a timestamp — with UTC and local time shown side by side.
A count of seconds since midnight UTC on 1 January 1970, called the epoch. It is a single number with no timezone attached, which is exactly why systems use it: two servers on opposite sides of the world agree on it without negotiating anything.
1700000000 is 14 November 2023 at 22:13:20 UTC. The number carries no clue about that, which is why logs full of timestamps need a converter.
Unix, Python and most databases use seconds. JavaScript and Java use milliseconds. Mixing them is the most common timestamp bug there is, and it fails in a distinctive way: seconds read as milliseconds land in January 1970, and milliseconds read as seconds land somewhere around the year 55,000.
The quick check is length. Ten digits is seconds; thirteen is milliseconds.
A signed 32-bit integer runs out on 19 January 2038, at which point it wraps to a negative number and dates jump back to 1901. Modern systems use 64-bit values and are unaffected, but old embedded systems and legacy databases are still being found and fixed.
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