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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.

Unix (seconds)
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Unix (milliseconds)
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ISO 8601
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UTC
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About this calculator

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

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.