ISO 8601 and RFC 3339 validator

RFC 3339 is a strict internet profile of ISO 8601: it requires the full date, time and T designator with a numeric offset (Z or ±hh:mm). ISO 8601 is broader — it also permits week dates (2024-W14-5), ordinal dates (2024-096) and omitted components. When an API rejects your string, it almost always wants RFC 3339 rather than anything-goes ISO.

Three usual suspects: a missing timezone (2024-04-05T11:34:38 carries no zone — JS reads it as local, Python as naive), the space-instead-of-T form (valid ISO, rejected by strict RFC 3339 parsers), and fractional digits beyond millisecond precision. Normalize to the 2024-04-05T11:34:38.000Z shape first.

Only if you include the offset. A trailing Z means UTC, +08:00 means that offset, and nothing at all means unspecified — every language fills the gap differently. The validator below flags missing offsets explicitly so you never ship an ambiguous string.

Parse to an instant, then divide milliseconds by 1000 — in JS Math.floor(Date.parse(iso)/1000), in Python int(datetime.fromisoformat(iso).replace(tzinfo=timezone.utc).timestamp()). The demo below does exactly this and warns on offset-less input instead of guessing.

Yes — pure page JavaScript, nothing uploaded. Safe for timestamps inside signed URLs, tokens and production payloads.