What is Unix epoch time?

A Unix timestamp (also called epoch time or POSIX time) is the number of seconds that have elapsed since 1970-01-01T00:00:00 UTC, ignoring leap seconds. It is a single number that represents the same instant everywhere on Earth, regardless of timezone.

The Unix time system was defined in the early 1970s. Its designers picked 1970-01-01 00:00:00 UTC as a simple, easy-to-remember reference point that was close to when Unix was being developed. The date itself has no special cosmic significance — it just needed to be a fixed, agreed-upon moment.

Both measure the same thing, but at different scales. A seconds timestamp has about 10 digits (e.g. 1754136000) and counts whole seconds. A milliseconds timestamp has about 13 digits (e.g. 1754136000000) and includes sub-second precision — its numeric value is exactly 1000× the seconds value.

Count the digits: ~10 digits is seconds, ~13 is milliseconds, ~16 is microseconds and ~19 is nanoseconds. The converter on this site auto-detects the unit by checking which interpretation produces a plausible date between 1960 and 2180, so you rarely need to think about it.

Classic 32-bit systems store the timestamp as a signed 32-bit integer, which overflows on 2038-01-19T03:14:08Z — the moment the value exceeds 2,147,483,647. Modern systems use 64-bit integers (or store milliseconds), which will not overflow for billions of years, so this mostly affects legacy 32-bit systems.

ISO 8601 is the international standard for representing dates and times as text, e.g. 2025-08-02T12:00:00Z. Epoch time and ISO 8601 describe the same instants: epoch is a machine-friendly number of seconds since 1970, while ISO 8601 is a human-readable string. Convert between them on the ISO 8601 page.

Excel and Google Sheets store dates as serial numbers — days since 1899-12-30 (or 1904-01-01 on the macOS 1904 system). A value like 45000.5 means noon on day 45,000. The Excel converter page handles these, including the famous 1900 leap-year bug.

Yes. Any instant before 1970-01-01T00:00:00Z has a negative timestamp, since the count runs backwards from the epoch. This converter handles negative timestamps (for example 1969 dates) automatically.

No. Everything runs in your browser — your timestamps never leave your device. There are no analytics, no tracking, no cookies, and no server-side logging. The only storage used is your browser’s localStorage, which remembers your timezone preference.

JavaScript numbers are exact up to 2^53 (about 9 × 10^15), so timestamps in seconds, milliseconds and microseconds are exact. Nanosecond values (about 1.7 × 10^18) exceed that range, so they are rounded slightly — close enough for display, but pair with a big-integer library if you need exact nanosecond math.