Epoch / Unix timestamp converter

It's a counter: the number of seconds elapsed since exactly midnight on the 1st of January 1970, in UTC. That moment is called the epoch. It was chosen by Ken Thompson and Dennis Ritchie when they were building the first Unix systems at Bell Labs in the early 1970s — they needed a simple integer to represent time, and "a few years before now" fit comfortably in a signed 32-bit integer.

Because it's UTC-anchored and timezone-free, two systems anywhere in the world will agree on what an epoch number means. It's only when you display it that timezone enters the picture.

Unix originally counted seconds. But modern programming languages (JavaScript, Java, .NET) defaulted to milliseconds when they were designed in the 1990s, because they needed sub-second precision for event timestamps. So you'll see two conventions in the wild:

• 10 digits (e.g. 1 700 000 000) — seconds. Used by Unix tools, syslog, MySQL TIMESTAMP, JWT exp claims.
• 13 digits (e.g. 1 700 000 000 000) — milliseconds. Used by Date.now() in JavaScript, System.currentTimeMillis() in Java, Postgres extract(epoch from … × 1000), Firestore Timestamps.

The cutoff is 10¹² (1 000 000 000 000). Anything bigger is definitely milliseconds — a 10¹² seconds value would be year 33658.

The epoch number itself doesn't care about timezones. The number 1 700 000 000 points to the same instant in Tokyo, London and New York — only the wall clock shows it differently.

This tool lets you pick the timezone the result is displayed in. By default it auto-detects yours via Intl.DateTimeFormat().resolvedOptions().timeZone, but you can type any IANA timezone name (e.g. Europe/Berlin, America/Argentina/Buenos_Aires) into the field.

Daylight saving time is handled automatically by the browser's Intl API — the same epoch can land at different wall-clock times depending on whether DST was in effect at that moment.

32-bit signed integers can hold values up to 2 147 483 647 — which corresponds to 03:14:07 UTC on Tuesday, 19 January 2038. After that, 32-bit Unix time overflows. Old C code that still uses time_t as a 32-bit type will wrap to negative numbers (circa 1901), with predictable disasters.

Modern systems use 64-bit time_t (signed), which pushes the rollover out to year 292 277 026 596 — well past the expected heat death of the universe. JavaScript stores epoch in a 64-bit float (millisecond precision), which is safe through year ≈275 760. So unless you're maintaining a 32-bit C codebase from 1990, you're unlikely to be bitten.