What Unix time is
Unix time is the number of seconds elapsed since 00:00:00 UTC on 1 January 1970, a moment known as the epoch. It ignores timezones and daylight saving entirely — a Unix timestamp refers to the same instant everywhere on Earth, which is exactly why systems use it for logs, databases and APIs. The human-readable date you see is a rendering of that instant into a particular timezone, not a different value.
Seconds or milliseconds?
Unix conventionally counts whole seconds, and a ten-digit number is almost certainly seconds. JavaScript, Java and most JSON APIs count milliseconds, giving a thirteen-digit number. Mixing them is one of the most common date bugs there is: interpreting milliseconds as seconds throws the date roughly fifty thousand years into the future, and the reverse lands you in January 1970. This converter accepts either and tells you which it detected.
The year 2038 problem
A signed 32-bit integer can hold Unix time only up to 03:14:07 UTC on 19 January 2038, after which it overflows to a negative number and the date reads as December 1901. Modern systems use 64-bit integers and are unaffected, but 32-bit timestamps still exist in embedded devices, older file formats and legacy database columns. If you work with long-dated records — mortgages, warranties, retention schedules — it is worth checking now rather than in 2037.
Leap seconds
Unix time deliberately pretends leap seconds do not exist: every day contains exactly 86,400 seconds. When a leap second is inserted, the Unix clock repeats a value rather than counting past it. This keeps date arithmetic simple at the cost of being very slightly out of step with astronomical time, which is the right trade for almost every application.