Date and Time on the Internet: Timestamps with additional information

1.1. Scope

This document defines an extension syntax for timestamps as specified in [RFC3339] that has the following properties:¶

• The extension suffix is completely optional, making existing [RFC3339] timestamps compatible with this format.¶
• The format is compatible with the pre-existing popular syntax for attaching time zone names to timestamps [JAVAZDT].¶
• The format provides a generalized way to attach additional information to the timestamp.¶

We refer to this format as the Internet Extended Date/Time Format (IXDTF).¶

This document does not address extensions to the format where the semantic result is no longer a fixed timestamp that is referenced to a (past or future) UTC time. For instance, it does not address:¶

• 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.¶
• The use of timescales different from UTC, such as International Atomic Time (TAI).¶

However, additional information augmenting a fixed timestamp may be sufficient to detect an inconsistency between intention and the actual information in the timestamp, such as between the UTC offset and time zone name. For instance, inconsistencies might arise because of:¶

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

While the information available in an IXDTF string is not generally sufficient to resolve an inconsistency, it may be used to initiate some out of band processing to obtain sufficient information for such a resolution.¶

In order to address some of the requirements implied here, future related specifications might define syntax and semantics of strings similar to [RFC3339]. Note that the extension syntax defined in the present document is designed in such a way that it can be useful for such specifications as well.¶

1.2. Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

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 Frames Service [IERS]. From 1972 through 1987, UTC was maintained entirely by 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, an earlier timescale UTC was designed to be a useful successor for.¶

ABNF:

Augmented Backus-Naur Form, a format used to represent permissible strings in a protocol or language, as defined in [RFC5234]. The rules defined in Appendix B of [RFC5234] are imported implicitly.¶

Internet Extended Date/Time Format (IXDTF):

The date/time format defined in Section 4 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 New York in the wintertime is 5 hours behind UTC, so its UTC offset is "-05:00".¶

Z:

A suffix which, when applied to a time, denotes a UTC offset of 00:00; often spoken "Zulu" from the ICAO phonetic alphabet representation of the letter "Z". (Definition from Section 2 of [RFC3339].)¶

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", 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 [TZDB][BCP175]. 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 [TZDB-NAMING].¶

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 to apply that are current at the time of interpretation: 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 RFC 3339 timestamp, for example:¶

2022-07-08T00:14:07+08:45[+08:45]

¶

An offset in the suffix that does not repeat the offset of the timestamp is inconsistent (see Section 3.4).¶

Although serialization with offset time zones is supported in this document for backwards compatibility with java.time.ZonedDateTime [JAVAZDT], 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 which 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). But 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 timezone Europe/Paris.¶

CLDR:

Common locale data repository [CLDR], a project of the Unicode Consortium to provide locale data to applications.¶

For more information about timescales, see Appendix E of [RFC1305], Section 3 of [ISO8601:1988], and the appropriate ITU documents [ITU-R-TF.460-6].¶

2.1. Background

Section 4.3 of [RFC3339] states that an offset given as Z or +00:00 implies that "UTC is the preferred reference point for the specified time". The offset -00:00 is provided as a way to express that "the time in UTC is known, but the offset to local time is unknown".¶

This convention mirrors a similar convention for date/time information in email headers, described in Section 3.3 of [RFC5322] and introduced earlier in Section 3.3 of [RFC2822]. This email header convention is in actual use, while its adaptation into [RFC3339] always was handicapped by the fact that [ISO8601:2000] does not actually allow -00:00.¶

Implementations that needed to express the semantics of -00:00 therefore tended to use Z instead.¶

2.2. Update to RFC 3339

This specification updates Section 4.3 of [RFC3339], aligning it with the actual practice of interpreting the offset Z to mean the same as-00:00: "the time in UTC is known, but the offset to local time is unknown".¶

A revised Section 4.3 of [RFC3339] with the update could read as follows:¶

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 [ISO8601:2000] and therefore is less interoperable; Section 3.3 of [RFC5322] 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.¶

2.3. Notes

Note that the semantics of the local offset +00:00 is not updated; this retains the implication that UTC is the preferred reference point for the specified time.¶

Note also that the fact that [ISO8601:2000] does not allow -00:00 as a local offset reduces the level of interoperability that can be achieved in using this feature; the present specification however does not formally deprecate this syntax. With the update to RFC 3339, the local offset Z can be used in its place.¶

3.1. Format of Extended Information

The format allows implementations to specify additional important information in addition to a bare [RFC3339] timestamp.¶

This is done by defining tags, each with a key and a value separated by an equals sign. The value of a tag can be one or more items delimited by hyphen/minus signs.¶

Applications can build an informative timestamp suffix using any number of these tags.¶

Keys are lower-case only. Values are case-sensitive unless otherwise specified.¶

See Section 3.3 for the handling of inconsistent information in a suffix.¶

3.2. Registering Keys for Extended Information Tags

Suffix tag keys are registered by supplying the information specified in this section. This information is modeled after that specified for the media type registry [RFC6838]; if in doubt, the provisions of this registry should be applied analogously.¶

Key Identifier:

The key (conforming to suffix-key in Section 4.1)¶

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 and 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.¶

Key names that start with an underscore are intended for experiments in controlled environments and cannot be registered; such keys MUST NOT be used for interchange and MUST be rejected by implementations not specifically configured to take part in such an experiment. See [BCP178] for a discussion about the danger of experimental keys leaking out to general production and why that MUST be prevented.¶

3.3. Optional Generation, Elective vs. Critical Consumption

For the IXDTF format, suffix tags are always optional: They can be added or left out as desired by the generator of the string. (An application might require the presence of specific suffix tags, though.)¶

Without further indication, suffix tags are also elective: The recipient is free to ignore any suffix tag included in an IXDTF string. Reasons might include that the recipient does not implement (or know about) the specific suffix key, or that it does recognize the key but cannot act on the value provided.¶

A suffix tag may also indicate that it is critical: The recipient is advised that it MUST NOT act on the Internet Extended Date/Time Format (IXDTF) string unless it can process the suffix tag as specified. A critical suffix tag is indicated by following its opening bracket with an exclamation mark (see critical-flag in Section 4.1).¶

For example, IXDTF strings such as:¶

2022-07-08T00:14:07+01:00[Europe/Paris]

are internally inconsistent (see Section 3.4), because Europe/Paris did not use a time zone offset of +01:00 in July 2022. The time zone hint given in the suffix tag is elective, though, so the recipient is not required to act on the inconsistency; it can treat the Internet Date/Time Format string as if it were:¶

2022-07-08T00:14:07+01:00

Similarly, an unknown suffix may be entirely ignored:¶

2022-07-08T00:14:07+01:00[knort=blargel]

(assuming that the recipient does not understand the suffix key knort).¶

In contrast to this elective use of a suffix tag,¶

2022-07-08T00:14:07+01:00[!Europe/Paris]
2022-07-08T00:14:07Z[!u-ca=chinese][u-ca=japanese]
2022-07-08T00:14:07Z[u-ca=chinese][!u-ca=japanese]
2022-07-08T00:14:07Z[!knort=blargel]

each have an internal inconsistency or an unrecognized suffix key/value that are marked as critical, so a recipient MUST treat these IXDTF strings as erroneous. This means that the application MUST reject the data, or perform some other error handling, such as asking the user how to resolve the inconsistency (see Section 3.4).¶

Note that applications MAY also perform additional processing on inconsistent or unrecognized elective suffix tags, such as asking the user how to resolve the inconsistency. While they are not required to do so with elective suffix tags, they are required to reject or perform some other error handling when encountering inconsistent or unrecognized suffix tags marked as critical.¶

An application that encounters duplicate use of a suffix key in elective suffixes and does not want to perform additional processing on this inconsistency MUST choose the first suffix that has that key, i.e.,¶

2022-07-08T00:14:07Z[u-ca=chinese][u-ca=japanese]
2022-07-08T00:14:07Z[u-ca=chinese]

are then treated the same.¶

3.4. Inconsistent time-offset/Time-Zone Information

An RFC 3339 timestamp can contain a time-offset value that indicates the offset between local time and UTC (see Section 4 of [RFC3339], noting that Section 2 of the present specification updates Section 4.3 of [RFC3339]).¶

The information given in such a time-offset value can be inconsistent with the information provided in a time zone suffix for an IXDTF timestamp.¶

For example, a calendar application could store an IXDTF string representing a far-future meeting in a particular time zone. If that time zone's definition is subsequently changed to abolish daylight saving time, IXDTF strings that were originally consistent may now be inconsistent.¶

In case of inconsistent time-offset and time zone suffix, if the critical flag is used on the time zone suffix, an application MUST act on the inconsistency. If the critical flag is not used, it MAY act on the inconsistency. Acting on the inconsistency may involve rejecting the timestamp, or resolving the inconsistency via additional information such as user input and/or programmed behavior.¶

For example, the IXDTF timestamps in Figure 1 represent 00:14:07 UTC, indicating a local time with a time-offset of +00:00. However, because Europe/London used offset +01:00 in July 2022, the timestamps are inconsistent:¶

2022-07-08T00:14:07+00:00[!Europe/London]
2022-07-08T00:14:07+00:00[Europe/London]

As per Section 4.3 of [RFC3339] as updated by Section 2, IXDTF timestamps may also forego indicating local time information in their [RFC3339] part by using Z instead of a numeric time zone offset. The IXDTF timestamps in Figure 2 (which represent the same instant in time as the strings in Figure 1) are not inconsistent because they do not assert any particular local time nor local offset in their [RFC3339] part. Instead, applications that receive these strings can calculate the local offset and local time using the rules of the time zone suffix, such as Europe/London in the example below.¶

2022-07-08T00:14:07Z[!Europe/London]
2022-07-08T00:14:07Z[Europe/London]

Note that -00:00 may be used instead of Z, because they have the same meaning according to Section 2, but -00:00 is not allowed by [ISO8601:2000] so Z is preferred.¶

4.1. ABNF

The following rules extend the ABNF syntax defined in [RFC3339] in order to allow the inclusion of an optional suffix.¶

The Internet Extended Date/Time Format (IXDTF) is described by the rule date-time-ext.¶

date-time and time-numoffset are imported from Section 5.6 of [RFC3339], ALPHA and DIGIT from Appendix B.1 of [RFC5234].¶

time-zone-initial = ALPHA / "." / "_"
time-zone-char    = time-zone-initial / DIGIT / "-" / "+"
time-zone-part    = time-zone-initial *time-zone-char
                    ; but not "." or ".."
time-zone-name    = time-zone-part *("/" time-zone-part)
time-zone         = "[" critical-flag
                        time-zone-name / time-numoffset "]"

key-initial       = lcalpha / "_"
key-char          = key-initial / DIGIT / "-"
suffix-key        = key-initial *key-char

suffix-value      = 1*alphanum
suffix-values     = suffix-value *("-" suffix-value)
suffix-tag        = "[" critical-flag
                        suffix-key "=" suffix-values "]"
suffix            = [time-zone] *suffix-tag

date-time-ext     = date-time suffix

critical-flag     = [ "!" ]

alphanum          = ALPHA / DIGIT
lcalpha           = %x61-7A

Note that a time-zone is syntactically similar to a suffix-tag, but does not include an equals sign. This special case is only available for time zone tags.¶

time-zone-name is intended to be the name of an IANA Time Zone. As generator and recipient may be using different revisions of the Time Zone Database, recipients may not be aware of such an IANA Time Zone name and should treat such a situation as any other inconsistency.¶

4.2. Examples

Here are some examples of Internet Extended Date/Time Format (IXDTF).¶

1996-12-19T16:39:57-08:00

Figure 4 represents 39 minutes and 57 seconds after the 16th hour of December 19th, 1996 with an offset of -08:00 from UTC. Note that this is the same instant in time as 1996-12-20T00:39:57Z, expressed in UTC.¶

1996-12-19T16:39:57-08:00[America/Los_Angeles]

Figure 5 represents the exact same instant as the previous example but additionally specifies the human time zone associated with it ("Pacific Time") for time-zone-aware implementations to take into account.¶

1996-12-19T16:39:57-08:00[America/Los_Angeles][u-ca=hebrew]

Figure 6 represents the exact same instant, but it informs calendar-aware implementations (see Section 5) that they should project it to the Hebrew calendar.¶

1996-12-19T16:39:57-08:00[_foo=bar][_baz=bat]

Figure 7, based on Figure 4, utilizes keys identified as experimental by a leading underscore to declare two additional pieces of information in the suffix; these can be interpreted by implementations that take part in the controlled experiment making use of these tag keys.¶

7.1. Excessive Disclosure

The ability to include various pieces of ancillary information with a timestamp might lead to excessive disclosure. An example for possibly excessive disclosure is given in Section 7 of [RFC3339]. Similarly, divulging information about the calendar system or the language of choice may provide more information about the originator of a timestamp than the data minimization principle would permit [DATA-MINIMIZATION]. More generally speaking, generators of IXDTF timestamps need to consider whether information to be added to the timestamp is appropriate to divulge to the recipients of this information, and need to err on the side of minimizing such disclosure if the set of recipients is not under control of the originator.¶

7.2. Data Format Implementation Vulnerabilities

As usual when extending the syntax of a data format, this can lead to new vulnerabilities in implementations parsing and processing the format. No considerations are known for the IXDTF syntax that would pose concerns that are out of the ordinary.¶

7.3. Operating with Inconsistent Data

Information provided in the various parts of an IXDTF string may be inconsistent in interesting ways, both with the extensions defined in this specification (see for instance Section 3.4) and with future extensions still to be defined. Where consistent interpretation between multiple actors is required for security purposes (e.g., where timestamps are embedded as parameters in access control information), only such extensions can be employed that have a defined resolution of such inconsistent data.¶