What a Unix timestamp is

A Unix timestamp is an integer that counts how much time has passed since a fixed instant: 1970-01-01 00:00:00 Coordinated Universal Time, often called the Unix epoch. The count does not include leap seconds in the way a civil clock does. For everyday logs and APIs, you can treat it as “seconds since that midnight UTC.”

The number 0 is the epoch itself. If you convert 0 in the Unix timestamp converter and leave the clock on UTC, the ISO line should read 1970-01-01T00:00:00.000Z. That check is useful because it does not depend on your time zone. Another familiar value is 1609459200, which is 2021-01-01 00:00:00 UTC.

Systems do not always agree on the unit. Unix tools, many databases, and most HTTP APIs store seconds. JavaScript’s Date.now() and several analytics products store milliseconds — the same instant multiplied by 1,000. If a converted date lands in the year 56000, you almost certainly fed milliseconds into a seconds field. If the date lands in 1970 when you expected this year, you may have done the reverse.

Negative timestamps are valid. They point at instants before 1970. Browser date objects have a finite range; a number with too many digits will fail, and the converter will say so instead of inventing a clock.

The timestamp is not a calendar date by itself. It is a point on a timeline. Printing it as “Tuesday afternoon” requires a time zone. UTC is the zone the epoch is defined in. Local time is whatever this device is set to. The converter labels which one you asked for so the same integer is not mistaken for two different events.

When you press Now, the page reads the clock in this browser and writes the current count. If that number disagrees with another machine, compare the device clocks first. The tool does not call a network clock.