Reference

UTC seconds as decimal text

Encode a whole-second timestamp as a decimal string.

UTC seconds as decimal text

Time rules
Count whole seconds since 1970-01-01T00:00:00Z. Each day has 86,400 seconds; leap seconds are not represented.
Supported values are integers from 0 to 253402300799 inclusive (1970 through 9999).
There are no fractional seconds, local-time offsets or negative values. Reject unsupported values rather than round or guess.

Representation
Use a JSON string containing the integer in base 10, using digits 0-9.
Use "0" for zero. Otherwise no leading zeroes, signs, whitespace or extra characters are allowed.
Example: "1791722096" represents 2026-10-11T12:34:56Z.