# Free online Unix timestamp calculator

> A Unix timestamp is the number of seconds since January 1, 1970 UTC. This calculator converts between Unix timestamps and human-readable dates in both directions.

Convert a Unix timestamp to a human-readable date and time, or convert any date and time to its Unix timestamp in seconds and milliseconds.

**Interactive version:** https://onlinecalculator.me/date-time/unix-timestamp/
**Category:** Date & Time

## How to use

1. Choose a conversion direction using the toggle at the top.
2. Enter a Unix timestamp (in seconds) or a local date and time.
3. Read the results: Unix seconds, Unix milliseconds, ISO 8601 string, and UTC string.
4. Press "Share with my numbers" to copy a link with your input pre-filled.

## How the conversion works

**Timestamp to date:** Multiply the input seconds by 1,000 to get milliseconds, construct a JavaScript `Date` object, then format it as ISO 8601 and UTC.

**Date to timestamp:** Parse the local datetime string as a `Date` object, then divide `date.getTime()` (milliseconds since epoch) by 1,000 and floor it to get Unix seconds.

## Worked example

Unix timestamp: 1,751,630,400

- Unix milliseconds: 1,751,630,400,000
- ISO 8601: 2025-07-04T12:00:00.000Z
- UTC: Friday, 4 July 2025, 12:00:00 UTC

## Notes

- The calculator interprets the datetime-local input as your local time zone. The ISO and UTC outputs are always in UTC.
- Negative Unix timestamps represent dates before January 1, 1970.
- JavaScript's `Date` object internally uses 64-bit floats for milliseconds, giving reliable precision for all dates between roughly year 271,821 BCE and year 275,760 CE.

## Frequently asked questions

### What is a Unix timestamp?

A Unix timestamp (also called epoch time or POSIX time) is the number of seconds that have elapsed since 00:00:00 UTC on January 1, 1970. It is used widely in programming because it is a simple integer that does not depend on time zones or calendar systems.

### What is the difference between seconds and milliseconds timestamps?

Most Unix timestamps are in seconds. JavaScript's Date.now() and many APIs return milliseconds — a value 1,000 times larger. The calculator shows both so you can pick the right one for your system.

### What does ISO 8601 mean?

ISO 8601 is the international standard for date and time representation. An ISO 8601 string looks like 2025-07-04T12:00:00.000Z — year, month, day, T separator, hours, minutes, seconds, and a Z suffix indicating UTC.

### Why does my timestamp look different from what I expect?

Timestamps are always in UTC internally. The UTC display shows the absolute time; your local time may differ by your time zone offset. Enter a datetime-local value to get the timestamp for your local time zone.

### What is the maximum Unix timestamp?

A 32-bit signed integer can hold timestamps up to 2,147,483,647, which corresponds to January 19, 2038 — the "Year 2038 problem." Modern systems use 64-bit integers, supporting dates billions of years into the future.

## Sources

- [Unix time](https://en.wikipedia.org/wiki/Unix_time) — Wikipedia
- [Coordinated Universal Time (UTC)](https://en.wikipedia.org/wiki/Coordinated_Universal_Time) — Wikipedia

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