What Is a Unix Timestamp (and Why Does It Start in 1970)?

A Unix timestamp — also called epoch time — is just an integer: the number of seconds that have elapsed since midnight UTC on January 1, 1970. No timezone, no calendar, no month or day fields to get wrong. That plainness is the entire point.

Why 1970

There's no deep meaning behind the date — it was simply chosen as a fixed reference point by the designers of Unix in the early 1970s, close to when the system was actually being built. What matters isn't the specific date but that everyone agreed on the same zero point, so any two systems computing a timestamp are counting from the same instant.

Why store time as a single number at all

Once time is a plain integer, comparing two moments is just comparing two numbers, and finding the duration between them is just subtraction — no calendar arithmetic, no daylight-saving edge cases, no ambiguity about which timezone a stored date was meant to be in. That's why timestamps are the default internally in databases, log files, and APIs: the number only gets converted to a human-readable date at the point where a person actually needs to read it.

Seconds vs milliseconds — the common gotcha

The Unix epoch itself is defined in seconds, but plenty of systems — JavaScript's Date.now() among them — report milliseconds instead, which is the same epoch just counted 1,000 times faster. A ten-digit timestamp is seconds; a thirteen-digit one is milliseconds. Feeding a millisecond value into something expecting seconds lands you on a date roughly 50,000 years in the future, which is a fast way to notice the mismatch.

The Year 2038 problem

Older systems that stored a Unix timestamp as a signed 32-bit integer can only count up to January 19, 2038, before the value overflows and wraps around to a negative number — interpreted as a date back in 1901. It's the timestamp equivalent of the Y2K bug. Most modern systems have already moved to 64-bit timestamps, which push the same problem out billions of years, but it's the reason you'll still see the date mentioned as a real deadline for anything running on older 32-bit infrastructure.

The practical rule

Treat a Unix timestamp as the unambiguous, storage-friendly form of a moment in time, and convert it to a human-readable date only at the point where a person needs to read it — and check whether you're holding seconds or milliseconds before you do the conversion.

Our Unix Timestamp Converter converts between epoch time and a readable date, in local time or UTC, instantly in your browser.