2026-06-10
Unix timestamp seconds vs milliseconds
Learn how to tell whether a timestamp is in seconds or milliseconds and why the difference matters.
Unix timestamp confusion usually comes down to seconds versus milliseconds. Backend systems often use seconds, while JavaScript dates use milliseconds.
A 10-digit value such as 1735689600 is commonly seconds. A 13-digit value such as 1735689600000 is commonly milliseconds. This is a useful heuristic, not a formal guarantee.
Try both units with the Unix Timestamp Converter and the Milliseconds Timestamp Converter when debugging an unfamiliar value.
Common symptoms
If a date lands in 1970, milliseconds may have been interpreted as seconds or seconds may have been interpreted as milliseconds. If a date lands far in the future, check the unit again.
API contract advice
Document the unit explicitly in API fields. Names such as createdAtUnixSeconds or createdAtMs are less ambiguous than timestamp.