Unix time counts seconds since 00:00:00 UTC on 1 January 1970, excluding leap seconds in common implementations.
Seconds and milliseconds
Many systems use seconds, producing values around ten digits today. JavaScript dates use milliseconds, typically thirteen digits.
Time zones
A timestamp represents an instant. A displayed date depends on the chosen time zone and locale.
ISO 8601
ISO strings such as 2026-07-12T08:30:00.000Z are explicit and portable. The Z suffix indicates UTC.
Practical advice
Store instants in a consistent format and apply local time zones only when presenting them to users.