• The extension suffix is completely optional, making existing timestamps compatible with this format.
• The format is compatible with the pre-existing popular syntax for attaching time zone names to timestamps .
• The format provides a generalized way to attach additional information to the timestamp.

• future time given as a local time in some specified time zone, where changes to the definition of that time zone (such as a political decision to enact or rescind daylight saving time) affect the instant in time represented by the timestamp;
• "floating time", i.e., a local time without information describing the UTC offset or time zone in which it should be interpreted; or
• the use of timescales different from UTC, such as International Atomic Time (TAI).

• political decisions, as discussed above,
• updates to time zone definitions being applied at different times by timestamp producers and receivers, or
• errors in programs producing and consuming timestamps.

UTC:

Coordinated Universal Time, as maintained since 1988 by the Bureau International des Poids et Mesures (BIPM) in conjunction with leap seconds as announced by the International Earth Rotation and Reference Systems Service . From 1972 through 1987, UTC was maintained entirely by the Bureau International de l'Heure (BIH). Before 1972, UTC was not generally recognized, and civil time was determined by individual jurisdictions using different techniques for attempting to follow Universal Time based on measuring the rotation of the earth. UTC is often mistakenly referred to as GMT (Greenwich Mean Time), an earlier timescale for which UTC was designed to be a useful successor.

ABNF:

Augmented Backus-Naur Form, a format used to represent permissible strings in a protocol or language, as defined in . The rules defined in are imported implicitly.

IXDTF:

Internet Extended Date/Time Format, the date/time format defined in of this document.

Timestamp:

An unambiguous representation of a particular instant in time.

UTC Offset:

Difference between a given local time and UTC, usually given in negative or positive hours and minutes. For example, local time in the city of New York, NY, USA in the wintertime in 2023 was 5 hours behind UTC, so its UTC offset was -05:00.

Z:

A suffix that, when applied to a time, denotes a UTC offset of 00:00; often pronounced "Zulu" from the ICAO phonetic alphabet representation of the letter "Z". (The definition is from ; see the International Civil Aviation Organization (ICAO) document for the phonetic alphabet mentioned.)

Time Zone:

A set of rules representing the relationship of local time to UTC for a particular place or region. Mathematically, a time zone can be thought of as a function that maps timestamps to UTC offsets. Time zones can deterministically convert a timestamp to local time. They can also be used in the reverse direction to convert local time to a timestamp, with the caveat that some local times may have zero or multiple possible timestamps due to nearby daylight saving time changes or other changes to the UTC offset of that time zone. Unlike the UTC offset of a timestamp, which makes no claims about the UTC offset of other related timestamps (and which is therefore unsuitable for performing local-time operations, such as "one day later"), a time zone also defines how to derive new timestamps based on differences in local time. For example, to calculate "one day later than this timestamp in San Francisco, California", a time zone is required because the UTC offset of local time in San Francisco can change from one day to the next.

IANA Time Zone:

A named time zone that is included in the Time Zone Database (often called tz or zoneinfo) maintained by IANA . Most IANA Time Zones are named for the largest city in a particular region that shares the same time zone rules, e.g., Europe/Paris or Asia/Tokyo . The rules defined for a named IANA Time Zone can change over time. The use of a named IANA Time Zone implies that the intent is for the rules that are current at the time of interpretation to apply: the additional information conveyed by using that time zone name is to change with any rule changes as recorded in the IANA Time Zone Database.

Offset Time Zone:

A time zone defined by a specific UTC offset, e.g., +08:45, and serialized using as its name the same numeric UTC offset format used in an timestamp, for example: An offset in the suffix that does not repeat the offset of the timestamp is inconsistent (see ). Although serialization with offset time zones is supported in this document for backwards compatibility with java.time.ZonedDateTime , use of offset time zones is strongly discouraged. In particular, programs MUST NOT copy the UTC offset from a timestamp into an offset time zone in order to satisfy another program that requires a time zone suffix in its input. Doing this will improperly assert that the UTC offset of timestamps in that location will never change, which can result in incorrect calculations in programs that add, subtract, or otherwise derive new timestamps from the one provided. For example, 2020-01-01T00:00+01:00[Europe/Paris] will let programs add six months to the timestamp while adjusting for summer time (daylight saving time). However, the same calculation applied to 2020-01-01T00:00+01:00[+01:00] will produce an incorrect result that will be off by one hour in the time zone Europe/Paris.

CLDR:

Common Locale Data Repository , a project of the Unicode Consortium to provide locale data to applications.

If the time in UTC is known, but the offset to local time is unknown, this can be represented with an offset of "Z". (The original version of this specification provided -00:00 for this purpose, which is not allowed by and therefore is less interoperable; describes a related convention for email, which does not have this problem). This differs semantically from an offset of +00:00, which implies that UTC is the preferred reference point for the specified time.

Key Identifier:

The key (conforming to suffix-key in )

Registration Status:

"Provisional" or "Permanent"

Description:

A very brief description of the key

Change Controller:

Who is in control of evolving the specification governing values for this key. This information can include email addresses of contact points, discussion lists, and references to relevant web pages (URLs).

Reference:

A reference. For permanent tag keys, this includes a full specification. For provisional tag keys, there is an expectation that some information is available even if that does not amount to a full specification; in this case, the registrant is expected to improve this information over time.