ISO 8601 Parser
Paste any ISO 8601 or RFC 3339 date string to see each component annotated, validate RFC 3339 compliance, and convert to a Unix timestamp.
What is ISO 8601?
When you see a date written as 2024-06-23T15:04:05Z in a log file, an API response, or a database record, that is ISO 8601 format. ISO 8601 is an international standard for writing dates and times as text in a way that is unambiguous across languages and cultures — no confusion between June 5 and May 6 depending on which country you are in.
The format always goes biggest unit first: year → month → day → hour → minute → second. This means ISO 8601 dates sort correctly as plain text — alphabetical order and chronological order are the same. That is why virtually every API, database, and log system uses it.
The T between the date and time is a literal separator defined by the standard. The Z at the end means UTC — so 15:04:05Z is 3:04 PM at UTC, not 3:04 PM in your local time. A numeric offset like +02:00 means the time shown is 2 hours ahead of UTC.
ISO 8601 vs RFC 3339
RFC 3339 is a strict subset of ISO 8601 used in internet protocols such as HTTP, Atom feeds, and JWT tokens. RFC 3339 is always valid ISO 8601, but not all ISO 8601 is valid RFC 3339. The key differences:
- Full date required — RFC 3339 requires YYYY-MM-DD. ISO 8601 allows year-only (2024) or year-month (2024-06).
- Full time required — RFC 3339 requires HH:MM:SS. ISO 8601 allows HH:MM without seconds.
- T separator required — RFC 3339 requires the letter T between date and time. ISO 8601 allows a space instead.
- Offset required — RFC 3339 requires Z or ±HH:MM. ISO 8601 allows a datetime with no offset (treated as local time).
In practice, if you are reading an API spec that says “RFC 3339 format,” you can safely write 2024-06-23T15:04:05Z and it will be accepted everywhere.
Frequently asked questions
What is ISO 8601?
ISO 8601 is an international standard for representing dates and times as text. It defines formats like 2024-06-23T15:04:05Z — the big-endian order (year first) makes dates sortable as plain strings, which is why nearly every API, database, and log system uses it.
What is RFC 3339 and how is it different from ISO 8601?
RFC 3339 is a stricter profile of ISO 8601 used in internet protocols. It requires a full date (YYYY-MM-DD), a full time (HH:MM:SS), the letter T as a separator (not a space), and a UTC offset (Z or ±HH:MM). ISO 8601 allows date-only strings, space separators, and omitting the offset — RFC 3339 does not. In practice most APIs produce RFC 3339, which is always valid ISO 8601.
What does the Z at the end mean?
The Z stands for "Zulu time," which is military/aviation notation for UTC. 2024-06-23T15:04:05Z means 15:04:05 exactly at UTC — no offset applied. It is equivalent to +00:00 but shorter. RFC 3339 allows both Z and +00:00.
What is the difference between +00:00 and Z?
Technically none — both mean UTC. RFC 3339 and ISO 8601 treat them as equivalent. Some systems prefer Z for compactness; others always emit +00:00. A few older systems distinguish them semantically (Z = unknown local zone vs +00:00 = explicitly UTC), but this is non-standard.
What does the fractional seconds part represent?
The digits after the decimal point in the seconds field represent sub-second precision. Three digits (.123) is milliseconds, six digits (.123456) is microseconds, nine digits is nanoseconds. Most systems work at millisecond precision. The standard allows any number of digits; trailing zeros are usually omitted.
Can ISO 8601 represent dates without a time?
Yes — 2024-06-23 is a valid ISO 8601 date-only string. RFC 3339 requires a time, so date-only strings are valid ISO 8601 but not RFC 3339. Many APIs that say they use RFC 3339 actually accept ISO 8601 date-only strings as well.
Why does the parser show an 'assumed UTC' warning?
When a datetime string has no timezone offset — like 2024-06-23T15:04:05 — it is technically a 'naive' local datetime with no specified timezone. To compute a Unix epoch value the tool assumes UTC, which it flags with a warning. If this represents a local time in a different timezone, the epoch would be off by the UTC offset of that zone.