Unix timestamp converter
A Unix timestamp counts seconds since 1 January 1970 UTC. Paste one and you get the date plus the same instant in every other common epoch.
How to use the unix timestamp converter
Two things trip people up regularly. The first is seconds versus milliseconds: JavaScript works in milliseconds and almost everything else in seconds, so a timestamp that decodes to 1970 is a millisecond value being read as seconds, and one that decodes to the year 55000 is the reverse. A ten-digit number is seconds and a thirteen-digit one is milliseconds, that rule holds for any date between 2001 and 2286. The second is the 2038 problem: a signed 32-bit timestamp overflows on 19 January 2038, and systems still storing time in a 32-bit integer will wrap to 1901. Most modern systems use 64 bits and are fine for about 292 billion years.
Questions
Ten digits is seconds, thirteen is milliseconds, for any date this century. If it decodes to 1970 you have milliseconds being read as seconds.
A signed 32-bit Unix timestamp overflows on 19 January 2038 and wraps to 1901. Sixty-four bit systems are unaffected.
It was a convenient recent round date when Unix time was defined in the early 1970s. There is no deeper reason.
No, everything here is UTC. Timestamps are stored in UTC precisely so they are unambiguous; converting to a local zone is a separate step and needs to know the zone.
No. Unix time pretends every day is exactly 86,400 seconds, which is why it is simple and why it is not quite the same as real elapsed time.