Quick answer
A Unix timestamp counts elapsed time from 1970-01-01 00:00:00 UTC, usually in seconds. JavaScript timestamps commonly use milliseconds, so confusing the two creates dates near 1970 or far in the future. The timestamp identifies an instant; a time zone affects only how that instant is displayed.
How to do it
- Determine whether the value contains seconds, milliseconds, or another unit.
- Convert the numeric value to an instant relative to the Unix epoch.
- Display both UTC and the intended local time zone, including the zone label.
Recognize the unit
Contemporary second timestamps have about 10 digits, while millisecond timestamps have about 13. Digit count is a useful clue but not a complete specification. APIs should document the unit explicitly, especially for negative or far-future dates.
Time zones and daylight saving
Unix time itself has no calendar time zone. Formatting the same instant in Hong Kong, London, and New York produces different clock readings, and daylight-saving rules depend on date and region. Include an offset or IANA zone when exchanging human schedules.
Common mistakes to avoid
- Multiplying a millisecond value by 1000 again.
- Appending a local-zone label to a UTC calculation.
- Using formatted local dates as unique event identifiers.
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