Unix Epoch Timestamps Explained: Why 1970, Leap Seconds & The 2038 Problem

A Unix timestamp (also called POSIX time or Epoch time) is an integer representing the exact number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC) on Thursday, January 1, 1970, excluding leap seconds.

For example, the Unix timestamp 1700000000 corresponds to Wednesday, November 14, 2023, 22:13:20 UTC. Today, virtually every computer operating system, database (PostgreSQL, MySQL), message broker (Kafka), and network protocol measures time using this single integer coordinate.

The origin date was established by Unix pioneers Ken Thompson and Dennis Ritchie at Bell Labs during the development of early Unix on the PDP-11 computer in the early 1970s. They needed a convenient recent round date to serve as time zero.

An earlier version of Unix defined the epoch as January 1, 1971, with system time ticking at 60 Hz. Because 32-bit registers would overflow in under 2.5 years at 60 Hz, the specification was updated to count one-second intervals starting from January 1, 1970 UTC, expanding the operational lifespan of 32-bit systems to over 68 years.

Legacy 32-bit systems store the Unix timestamp as a signed 32-bit integer (int32_t). The maximum value a signed 32-bit integer can hold is 2^31 - 1 = 2,147,483,647.

At 03:14:07 UTC on Tuesday, January 19, 2038, that integer reaches its absolute limit. On the very next second, the integer overflows into the negative realm: -2,147,483,648, causing affected computer systems to think the current date is December 13, 1901!

Modern 64-bit operating systems use int64_t, which pushes the overflow point out to 292 billion years into the future — comfortably exceeding the lifespan of the universe.

Developers often need to inspect raw millisecond or second timestamps returned by API payloads. You can convert timestamps to human-readable dates and back with our browser-native Timestamp Converter.