How Unix time works
Unix time counts the seconds elapsed since 00:00:00 UTC on 1 January 1970, a moment called the epoch. Leap seconds are ignored, so every day is treated as exactly 86,400 seconds; this keeps the arithmetic simple at the cost of drifting a few seconds from astronomical time. Because a timestamp is just one number with no time zone baked in, the same value means the same instant everywhere in the world, which is why databases, log files, and APIs all use it. The only ambiguity is the unit: a 10-digit value like 1750000000 is seconds, while a 13-digit value like 1750000000000 is milliseconds, the form JavaScript uses.
Timestamps have a famous expiry date. Systems that store Unix time in a signed 32-bit integer run out of room at 2,147,483,647 seconds, which lands at 03:14:07 UTC on 19 January 2038. One second later the counter overflows and wraps around to December 1901, a failure mode often compared to the year 2000 bug. Mainstream operating systems and languages have long since moved to 64-bit time values, which are good for hundreds of billions of years, but embedded devices, old file formats, and legacy databases written against 32-bit time can still misbehave as the deadline approaches.
Frequently asked questions
What is Unix time?
Unix time is the number of seconds elapsed since 00:00:00 UTC on 1 January 1970, known as the epoch. It ignores leap seconds, so every day counts as exactly 86,400 seconds. Because it is a single number with no time zone attached, it is the standard way computers store and exchange points in time.
Is my timestamp in seconds or milliseconds?
Count the digits. Timestamps for current dates have 10 digits in seconds and 13 digits in milliseconds. JavaScript's Date.now() returns milliseconds, while most Unix tools and APIs use seconds. This converter treats values of a trillion or more as milliseconds and everything smaller as seconds.
What is the year 2038 problem?
Systems that store Unix time in a signed 32-bit integer can only count up to 2,147,483,647 seconds, which is reached at 03:14:07 UTC on 19 January 2038. One second later the value overflows and wraps to a date in 1901. Modern systems use 64-bit time values, but older embedded devices and file formats may still be affected.