What is the Unix epoch? Why computers count seconds from January 1, 1970

Timestamp 0 in Unix time is 00:00:00 UTC on January 1, 1970, the Unix epoch. Every Unix timestamp is the number of seconds that have elapsed since that moment, not counting leap seconds. It's a beautifully simple system: one integer represents any point in time, timezone-independent, sortable, and mathematically trivial to compare.

Unix timestamp
0
Seconds since epoch
ISO 8601
1970-01-01T00:00:00.000Z
Thu, 01 Jan 1970 00:00:00 GMT
Category
Fundamentals
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How this is calculated

The epoch was chosen pragmatically by Unix engineers at Bell Labs in the early 1970s. They needed a round number that was recent enough to keep timestamps small (fitting in a 32-bit signed integer) but far enough in the past to represent dates before the system was written. January 1, 1970, was convenient. The 32-bit signed integer limit (2,147,483,647 seconds) will overflow on January 19, 2038, the Year 2038 problem, which is why modern systems use 64-bit timestamps.

Verdict

Unix timestamps are the simplest way to store and compare points in time across systems. They're timezone-agnostic, monotonically increasing (except for leap seconds), and supported by every programming language and database. Store times as Unix timestamps, display them in the user's local timezone.

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Frequently asked questions

Why is the Unix epoch January 1, 1970?
It was a convenient round number close to when Unix was being developed at Bell Labs. There's no deeper technical reason. January 1, 1970, was a Thursday.