A Unix timestamp counts the seconds since the start of 1970, which makes it convenient for computers and unreadable for people. This converter goes both ways: paste the number from a log line, a database column or an API response and get the date, or type a date and get the number back. It works out on its own whether you pasted seconds or milliseconds — the usual source of an answer that is off by a factor of a thousand — and shows the moment in ISO, UTC and your own local time at once, along with how long ago it was.
By length. A timestamp in seconds has ten digits for any date between 2001 and 2286; the same moment in milliseconds has thirteen. This tool switches automatically on that, and shows you which it chose so you can correct it if the value is unusual.
Because a timestamp records an instant, not a wall clock. The same number is 09:00 in London and 18:00 in Seoul. The local line uses your device's time zone; the UTC and ISO lines are the same everywhere, which is why logs and APIs use them.
Systems that store the timestamp in a signed 32-bit integer run out of room on 19 January 2038 and wrap into negative numbers. Anything storing it in 64 bits — which is now most things, including this page — is unaffected for longer than the age of the universe.
Yes. Those are negative timestamps, counting seconds backwards from the epoch, and they convert correctly here.
No, and neither does any other Unix timestamp. The count deliberately pretends every day has exactly 86,400 seconds, which is why it stays simple to convert and why it drifts from astronomical time by a handful of seconds.