A Unix timestamp is unreadable, so for display and interchange it is written as text — and there are a few standard ways to do that. Three matter in practice, and they are easy to confuse.

ISO 8601

ISO 8601 is the broad international standard for writing dates and times. Its hallmark is the shape YYYY-MM-DDThh:mm:ss: a four-digit year, the T that separates the date from the time, and 24-hour time. A trailing Z (for "Zulu") means UTC; otherwise an offset like +05:30 or -08:00 states the zone. ISO 8601 is permissive — it allows week dates, ordinal dates, and various reduced forms — which makes it expressive but a little risky to parse blindly.

RFC 3339

RFC 3339 is a strict profile of ISO 8601, written specifically for timestamps on the internet. It nails down the ambiguities: a full date and time, a mandatory separator, and a required offset (Z or ±hh:mm). Most things that say "ISO 8601" in an API context actually mean RFC 3339. The two overlap heavily, but not perfectly — a valid ISO 8601 string is not always valid RFC 3339, and vice versa in edge cases — so when a spec says one, take it literally.

The sortability bonus

There is a quiet superpower in the YYYY-MM-DD ordering: because the most significant component comes first and every field is zero-padded to a fixed width, sorting these strings lexicographically sorts them chronologically. 2023-01-09 comes before 2023-11-14 as plain text, no date parsing required. This is why log files and object keys so often use ISO-style timestamps.

The HTTP date is different

One format refuses to follow the pattern: the date in HTTP headers like Last-Modified and Date. Defined by RFC 9110 (the IMF-fixdate format), it looks like Tue, 14 Nov 2023 22:13:20 GMT — day name, day, English month abbreviation, time, and the literal GMT. It is fixed-width and always UTC, but it sorts as nonsense and needs its own parser. When you see that shape, you are looking at an HTTP date, not ISO 8601.

This converter shows all three side by side for any instant, so you can copy the exact form a given system expects.