Unix timestamps: seconds, milliseconds, and ISO 8601

Unix time counts from 1970-01-01T00:00:00Z. The unit is the trap: APIs usually use seconds (about 10 digits today). JavaScript Date.now() uses milliseconds (13 digits). Mix them and you land in 1970 or the 50th millennium.

How to tell them apart

If the absolute value is at least 10^12, it is almost certainly milliseconds. Alphzen’s Unix Timestamp Converter uses that rule and also accepts ISO 8601 strings.

UTC vs local

Store instants in UTC. Display them in the user’s zone. The converter shows both so a log line and a calendar picker can be compared without arguing about offset.

Recurring schedules are not timestamps. For those, use the Cron Expression Generator.