What Is a Unix Timestamp?
A clear explanation of Unix epoch time, seconds vs milliseconds, and how to convert timestamps to local and UTC dates.
Unix time in plain language
A Unix timestamp is the number of seconds (or sometimes milliseconds) that have elapsed since 00:00:00 UTC on 1 January 1970 — the Unix epoch. It is a compact, timezone-independent way to store instants in time.
APIs, databases, log systems, and JWTs often use Unix time because it sorts easily and avoids ambiguous local calendar formats.
Seconds vs milliseconds
Most classic Unix systems use seconds. Many JavaScript APIs (Date.now()) use milliseconds. A 10-digit value is usually seconds; a 13-digit value is usually milliseconds. Mixing them up shifts dates by orders of magnitude.
When converting, confirm the unit. Toolshelf’s Unix Timestamp Converter can auto-detect common lengths or let you choose seconds or milliseconds explicitly.
Converting to a readable date
To debug a timestamp, convert it to both local time and UTC. Local time helps humans; UTC helps compare systems across regions. ISO 8601 strings are a good interchange format once you need human-readable absolute times with offsets.