Unix seconds vs milliseconds: convert a timestamp with the right timezone

Identify the unit before converting

A Unix-style timestamp describes time relative to 1 January 1970 at 00:00 UTC. Many systems use seconds, while JavaScript dates use milliseconds. The same digits therefore describe different instants when interpreted in different units. Kervoni does not guess the unit from the number of digits. Read the source system’s field description and choose the matching Input format. A short example near the epoch is useful for checking the distinction before using an unfamiliar value from a log or export.

Compare the value 1000 in both modes

Open the Timestamp Converter, select Unix seconds, enter 1000 and choose Convert. The UTC result is 1970-01-01T00:16:40.000Z and the millisecond value is 1000000. Now select Unix milliseconds, re-enter 1000 and convert again. UTC becomes 1970-01-01T00:00:01.000Z, with seconds equal to 1. Changing the input format clears the input and old results. Always enter the value again after changing the mode rather than relying on the previous display.

Use an explicit offset for an ISO date

Select ISO 8601 and enter 2026-10-08T15:00:00+03:00. It represents the same instant as 2026-10-08T12:00:00Z; the UTC output is 2026-10-08T12:00:00.000Z. This interface requires a full date and time with T and either Z or an offset such as +03:00. A date with no offset, a date-only value or a timezone name is not the accepted input form. The engine rejects invalid calendar dates rather than treating 30 February as a valid date.

Read UTC and device-local time separately

The UTC line ends in Z. The local-time line is formatted using your device’s timezone and displays that timezone name. Two devices can show different local clock readings for the same millisecond value. Changing the website language changes presentation, not the device’s configured timezone. Record the unit and offset with a value you share. A local clock time written without its offset can be ambiguous around daylight-saving changes; this converter does not choose an intended local occurrence from a named-zone schedule.

Keep input precision and workflow limits clear

Seconds and milliseconds inputs must be integers; fractional numeric input is rejected. ISO input can include up to three fractional-second digits, and the output seconds can consequently be fractional. The field allows at most 64 characters and extreme values outside the supported date range are rejected. There are no microsecond, nanosecond or leap-second-input modes. Editing the value clears the old result, so convert again. Processing stays on your device; the utility does not update the originating log, database or event.

Sources

• MDN: JavaScript dates, timestamps and time zones
• Kervoni: Accepted timestamp inputs