What Is Unix Time?

Unix time counts seconds since January 1, 1970 UTC, giving computers a single, unambiguous, timezone-independent way to represent a moment in time.

Why computers don't just store "dates"

Human-readable dates are tangled up with timezones, calendars, and formatting conventions that differ around the world. Unix time sidesteps all of that by representing any moment as a single number: the count of seconds elapsed since a fixed reference point, January 1, 1970, 00:00:00 UTC — known as the Unix epoch.

Seconds vs. milliseconds — a classic bug source

Traditional Unix timestamps are in seconds, but many programming environments (notably JavaScript) work in milliseconds instead, which is 1,000 times larger. Mixing the two up produces dates that are either wildly in the past (near 1970) or absurdly far in the future — one of the most common and easy-to-spot timestamp bugs.

Timezones only matter at display time

A Unix timestamp itself has no timezone attached — it's a single global number. Timezone only becomes relevant when converting that number into a human-readable calendar date and clock time for display, since the same instant corresponds to a different local date/time depending on where in the world you're looking from.

The Year 2038 problem

Older systems that store Unix time as a signed 32-bit integer can only represent times up to January 19, 2038, after which the value overflows. Virtually all modern systems have moved to 64-bit representations, which push this limit far enough into the future to be a non-issue in practice.