Compact UTC seconds in base 36

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 36. Digits 0-9 have values 0-9 and lowercase letters a-z have values 10-35.
Use "0" for zero. Otherwise no leading zeroes, signs, whitespace, uppercase letters or extra characters are allowed.
Reject strings longer than eight characters and decoded integers outside the supported range.
Decode the base-36 integer, multiply by 1000 for a millisecond-based date library, and display the resulting UTC instant.
Example: "tmqtm8" decodes to 1791722096 seconds, representing 2026-10-11T12:34:56Z.

Compatibility
This is a distinct format. A field or dataset using it must explicitly reference this specification version. Do not infer it from the characters alone.