Function iso8601_from_millis 

pub fn iso8601_from_millis(clock: ClockTime) -> String

ms-since-epoch → YYYY-MM-DDTHH:MM:SSZ UTC string. Matches the ISO-8601 format accepted by mimir_core::parse’s timestamp loader so the string round-trips bit-perfect.

Uses the Howard-Hinnant proleptic-Gregorian algorithm (integer-only; no chrono dependency) for the date component.