Unix timestamp to ISO 8601

Turning epoch seconds into a date

Convert Unix seconds since 1970-01-01 UTC into an ISO 8601 UTC date-time. Choose this direction when an API or log contains a Unix timestamp and you need a human-readable UTC date. Enter seconds, optionally with one to three decimal digits for milliseconds. Values before the 1970 epoch are negative. The converter does not assume the number is milliseconds, microseconds or nanoseconds.

Unix time counts seconds since the UTC epoch; fractions of a second are preserved to milliseconds. The example 1704067200 is exactly midnight at the start of 2024 in UTC and becomes 2024-01-01T00:00:00.000Z. The trailing Z states the timezone explicitly; .000 shows zero milliseconds. If an input is roughly thirteen digits long for a present-day date, it is probably milliseconds and must first be divided by one thousand. For a concrete check, the example input 1704067200 produces 2024-01-01T00:00:00.000Z. Keep the original beside the result when comparing it with another application.

Two ways to express one instant

A Unix timestamp counts elapsed seconds from 1970-01-01T00:00:00Z. An ISO 8601 date-time names calendar fields and carries a timezone indicator, such as Z for UTC or an offset like +02:00. Both can identify one instant when the timezone is explicit. This converter keeps fractions to three decimal places, so millisecond precision is preserved in either direction. It is useful for API records, audit logs, scheduled jobs and debugging a timestamp copied from a database without relying on the computer’s local timezone.

Offsets, precision and calendar pitfalls

An explicit timezone is required for ISO input. Leap seconds and more than millisecond precision are outside this tool’s scope. An offset describes how clock time differs from UTC at that written instant; it is not the same as a named timezone with daylight-saving rules. The conversion applies the offset supplied in the input and displays UTC in ISO output. A date lacking a timezone is deliberately rejected because interpreting it as local or UTC could change the instant. Leap seconds such as 23:59:60 are outside the standard Date calculation used here. Fractions smaller than a millisecond are also outside scope, and very distant years are limited by the browser date range. Check the unit before pasting a timestamp: milliseconds are one thousand times larger than seconds.