Guides / FreeTools Lab

Unix Timestamps Explained for Everyday Debugging

Understand seconds, milliseconds, UTC and local time before comparing logs.

A Unix timestamp represents elapsed time from the Unix epoch. Confusing seconds with milliseconds is a common reason that a date appears far in the past or future.

Seconds and milliseconds

A ten-digit value is often seconds while a thirteen-digit value is often milliseconds, but inspect the source system rather than relying only on digit count.

UTC and local time

An ISO time ending in Z is UTC. A local display can differ by timezone while representing the same instant. Record the timezone when comparing browser logs and server logs.

Use a converter as a check

Convert both values and compare the human-readable date. If they differ unexpectedly, check units, timezone and clock synchronization before changing code.

Use the related browser tools as a quick check, then verify the result for your own workflow.