nx_iso8601.nx

iso8601.nx -- format + parse ISO 8601 date-time strings. The canonical machine-readable timestamp format: 2026-04-22T12:34:56Z 2026-04-22T12:34:56.123Z (milliseconds -- optional) Used by: JSON APIs (the convention), HTTP Last-Modified headers (RFC 7231 also accepts IMF-fixdate but ISO is simpler), file export headers, nishi-pages post dates, obligation deadlines, log timestamps. We support the "basic profile" of ISO 8601: YYYY-MM-DDTHH:MM:SS[.fff]Z (UTC only, 'Z' suffix) Longer time-offset variants (+01:00, etc.) are NOT produced; we always emit UTC. Parser accepts both 'Z' and an optional '+HH:MM' / '-HH:MM' offset that we convert to seconds. Composes civil_date.nx. Invariants: I1 Format output is exactly 20 bytes for seconds precision, 24 bytes for millisecond precision. I2 Parser rejects strings with unknown fields (e.g. week numbers) with ISO_ERR_FORMAT; we stay narrowly useful. I3 Round-trip exact for format(parse(s)) on strings we emit.