Internet Engineering Task Force (IETF) U. Sharma

Internet Engineering Task Force (IETF) U. Sharma
Request for Comments: 9557 Igalia, S.L.
Updates: 3339 C. Bormann
Category: Standards Track Universität Bremen TZI
ISSN: 2070-1721 April 2024

Date and Time on the Internet: Timestamps with Additional Information

Abstract

This document defines an extension to the timestamp format defined in
RFC 3339 for representing additional information, including a time
zone.

It updates RFC 3339 in the specific interpretation of the local
offset Z, which is no longer understood to "imply that UTC is the
preferred reference point for the specified time".

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9557.

Copyright Notice

Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.

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Table of Contents

1. Introduction
1.1. Scope
1.2. Definitions
2. Updating RFC 3339
2.1. Background
2.2. Update to RFC 3339
2.3. Notes
3. Internet Extended Date/Time Format (IXDTF)
3.1. Format of Extended Information
3.2. Registering Keys for Extended Information Tags
3.3. Optional Generation and Elective vs. Critical Consumption
3.4. Inconsistent time-offset and Time Zone Information
4. Syntax Extensions to RFC 3339
4.1. ABNF
4.2. Examples
5. The u-ca Suffix Key: Calendar Awareness
6. IANA Considerations
7. Security Considerations
7.1. Excessive Disclosure
7.2. Data Format Implementation Vulnerabilities
7.3. Operating with Inconsistent Data
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Contributors
Authors' Addresses

1. Introduction

Dates and times are used in a very diverse set of Internet
applications, all the way from server-side logging to calendaring and
scheduling.

Each distinct instant in time can be represented in a descriptive
text format using a timestamp. [ISO8601-1:2019] standardizes a
widely adopted timestamp format, an earlier version of which
[ISO8601:1988] formed the basis of the Internet Date/Time Format
[RFC3339]. However, this format allows timestamps to contain very
little additional relevant information. Beyond that, any contextual
information related to a given timestamp needs to be either handled
separately or attached to it in a non-standard manner.

This is a pressing issue for applications that handle each such
instant in time with an associated time zone name in order to take
into account events such as daylight saving time transitions. Many
of these applications attach the time zone to the timestamp in a non-
standard format, at least one of which is fairly well-adopted
[JAVAZDT]. Furthermore, applications might want to attach even more
information to the timestamp, including but not limited to the
calendar system in which it should be represented.

This document defines an extension to the timestamp format defined in
[RFC3339] for representing additional information, including a time
zone.

It updates [RFC3339] in the specific interpretation of the local
offset Z, which is no longer understood to "imply that UTC is the
preferred reference point for the specified time"; see Section 2.

1.1. Scope

[RFC3339] defines a syntax for timestamps to represent date and time
in the Internet. The present document defines an extension syntax
that achieves 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; or

* 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 the 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,

* updates to time zone definitions being applied at different times
by timestamp producers and receivers, or

* errors in programs 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, related
specifications in the future might define syntax and semantics of
strings similar to those described in [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 Systems Service [IERS]. 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
[RFC5234]. The rules defined in Appendix B of [RFC5234] are
imported implicitly.

IXDTF: Internet Extended Date/Time Format, 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 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
Section 2 of [RFC3339]; see the International Civil Aviation
Organization (ICAO) document [ICAO-PA] 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 [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 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 [RFC3339] 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 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 [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]. (Note: [RFC1305] was obsoleted by [RFC5905], which
no longer contains the Appendix E referenced here.)

2. Updating RFC 3339

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 that is 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] was always compromised by the fact that [ISO8601:2000] and
later versions do 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".

Section 4.3 of [RFC3339] is revised to 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.

Also note that the fact that [ISO8601:2000] and later do not allow
-00:00 as a local offset reduces the level of interoperability that
can be achieved in using this feature; however, the present
specification does not formally deprecate this syntax. With the
update to [RFC3339], the local offset Z should now be used in its
place.

3. Internet Extended Date/Time Format (IXDTF)

This section discusses desirable qualities of formats for the
timestamp extension suffix and defines the IXDTF format, which
extends [RFC3339] for use in Internet protocols.

3.1. Format of Extended Information

The format allows applications 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 lowercase 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 Types" 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, 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 and 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 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. However, the
time zone hint given in the suffix tag is elective, 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

| Note that, as per Section 2 (see also Section 3.4), the IXDTF
| string:
|
| 2022-07-08T00:14:07Z[Europe/Paris]
|
| does not exhibit such an inconsistency, as the local offset of
| Z does not imply a specific preferred time zone of
| interpretation. Using the Time Zone Database rules for Europe/
| Paris in the summer of 2022, it is equivalent to:
|
| 2022-07-08T02:14:07+02:00[Europe/Paris]

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 is 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,
that is,

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 and Time Zone Information

An [RFC3339] 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 an inconsistency between 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, where the first case is one for which the
application MUST act on the inconsistency (the time zone suffix is
marked critical) and the second case is one for which it MAY act on
the inconsistency (the time zone suffix is elective).

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

Figure 1: Inconsistent IXDTF Timestamps

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 in Figure 2, which
like Figure 1 has a case with a time zone suffix marked critical
(i.e., the intention is that the application must understand the time
zone information) and one marked elective, which then only is
provided as additional information.

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

Figure 2: No Inconsistency in IXDTF Timestamps

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] and later so Z is preferred.

4. Syntax Extensions to RFC 3339

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], and ALPHA and DIGIT are imported 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

Figure 3: ABNF Grammar of Extensions to RFC 3339

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.

The ABNF definition of time-zone-part matches "." and "..", which are
both explicitly excluded (see the note below on time-zone-part).

time-zone-name is intended to be the name of an IANA Time Zone. As a
generator and a 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.

| Note: At the time of writing, the length of each time-zone-part
| is limited to a maximum of 14 characters by the rules in
| [TZDB-NAMING]. One platform is known to enforce this limit,
| and a time zone name on another platform is known to exceed
| this limit. As the time-zone-name will ultimately have to be
| looked up in the local database, which therefore has control
| over the length, the time-zone-part production in Figure 3 is
| deliberately permissive.

4.2. Examples

This section contains some examples of Internet Extended Date/Time
Format (IXDTF).

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

Figure 4: RFC 3339 date-time with Time Zone Offset

Figure 4 represents 39 minutes and 57 seconds after the 16th hour of
December 19, 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: Adding a Time Zone Name

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

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

Figure 6: Projecting to the Hebrew Calendar

Figure 6 represents the exact same instant in time, but it informs
calendar-aware applications (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: Adding Experimental Tags

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.

5. The u-ca Suffix Key: Calendar Awareness

Out of the possible suffix keys, the suffix key u-ca is allocated to
indicate the calendar in which the date/time is preferably presented.

A calendar is a set of rules defining how dates are counted and
consumed by implementations. The set of suffix values allowed for
this suffix key is the set of values defined for the Unicode Calendar
Identifier [TR35]. [CLDR-LINKS] provides links to the most recent
information about [CLDR], both stable and specific development
stages.

6. IANA Considerations

IANA has created a registry called "Timestamp Suffix Tag Keys" in a
new registry group titled "Internet Date/Time Format". Each entry in
the registry shall consist of the information described in
Section 3.2. The initial contents of the registry are specified in
Table 1.

+============+==============+==============+============+=========+
| Key | Registration | Description | Change |Reference|
| Identifier | Status | | Controller | |
+============+==============+==============+============+=========+
| u-ca | Permanent | Preferred | IETF |Section 5|
| | | Calendar for | |of RFC |
| | | Presentation | |9557 |
+------------+--------------+--------------+------------+---------+

Table 1: Initial Contents of Timestamp Suffix Tag Keys Registry

The registration policy [BCP26] is "Specification Required" for
permanent entries and "Expert Review" for provisional ones. In the
second case, the experts are instructed to ascertain that a basic
specification does exist, even if not complete or published yet.

The experts are also instructed to be frugal in the allocation of key
identifiers that are suggestive of generally applicable semantics,
keeping them in reserve for suffix keys that are likely to enjoy wide
use and can make good use of the key identifier's conciseness. If
the experts become aware of key identifiers that are deployed and in
use, they may also initiate a registration on their own if they deem
such a registration can avert potential future collisions.

7. Security Considerations

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 (for instance, see 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 extensions that have a well-understood and shared
resolution of such inconsistent data can be employed.

8. References

8.1. Normative References

[BCP175] Best Current Practice 175,
<https://www.rfc-editor.org/info/bcp175>.
At the time of writing, this BCP comprises the following:

Lear, E. and P. Eggert, "Procedures for Maintaining the
Time Zone Database", BCP 175, RFC 6557,
DOI 10.17487/RFC6557, February 2012,
<https://www.rfc-editor.org/info/rfc6557>.

[BCP178] Best Current Practice 178,
<https://www.rfc-editor.org/info/bcp178>.
At the time of writing, this BCP comprises the following:

Saint-Andre, P., Crocker, D., and M. Nottingham,
"Deprecating the "X-" Prefix and Similar Constructs in
Application Protocols", BCP 178, RFC 6648,
DOI 10.17487/RFC6648, June 2012,
<https://www.rfc-editor.org/info/rfc6648>.

[BCP26] Best Current Practice 26,
<https://www.rfc-editor.org/info/bcp26>.
At the time of writing, this BCP comprises the following:

Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<https://www.rfc-editor.org/info/rfc3339>.

[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.

[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.

[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.

8.2. Informative References

[CLDR] Unicode CLDR, "Unicode CLDR Project",
<https://cldr.unicode.org>.

[CLDR-LINKS]
Unicode CLDR, "Stable Links Info",
<https://cldr.unicode.org/stable-links-info>.

[DATA-MINIMIZATION]
Arkko, J., "Emphasizing data minimization among protocol
participants", Work in Progress, Internet-Draft, draft-
arkko-iab-data-minimization-principle-05, 10 July 2023,
<https://datatracker.ietf.org/doc/html/draft-arkko-iab-
data-minimization-principle-05>.

[ICAO-PA] International Civil Aviation Organization, "Annex 10 to
the Convention on International Civil Aviation:
Aeronautical Telecommunications; Volume II Communication
Procedures including those with PANS status", 7th ed.,
July 2016, <https://store.icao.int/annex-10-aeronautical-
telecommunications-volume-ii-communication-procedures-
including-those-with-pans-status>.

[IERS] IERS, "International Earth Rotation Service Bulletins",
<https://www.iers.org/IERS/EN/Publications/Bulletins/
bulletins.html>.

[ISO8601-1:2019]
ISO, "Date and time -- Representations for information
interchange -- Part 1: Basic rules", ISO 8601-1:2019,
February 2019, <https://www.iso.org/standard/70907.html>.

[ISO8601:1988]
ISO, "Data elements and interchange formats -- Information
interchange -- Representation of dates and times",
ISO 8601:1988, June 1988,
<https://www.iso.org/standard/15903.html>. Also available
from <https://nvlpubs.nist.gov/nistpubs/Legacy/FIPS/
fipspub4-1-1991.pdf>.

[ISO8601:2000]
ISO, "Data elements and interchange formats -- Information
interchange -- Representation of dates and times",
ISO 8601:2000, December 2000,
<https://www.iso.org/standard/26780.html>.

[ITU-R-TF.460-6]
ITU-R, "Standard-frequency and time-signal emissions",
ITU-R Recommendation TF.460-6, February 2002,
<https://www.itu.int/rec/R-REC-TF.460/en>.

[JAVAZDT] Oracle, "Class DateTimeFormatter: ISO_ZONED_DATE_TIME",
<https://docs.oracle.com/javase/8/docs/api/java/time/
format/DateTimeFormatter.html#ISO_ZONED_DATE_TIME>.

[RFC1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation and Analysis", RFC 1305,
DOI 10.17487/RFC1305, March 1992,
<https://www.rfc-editor.org/info/rfc1305>.

[RFC2822] Resnick, P., Ed., "Internet Message Format", RFC 2822,
DOI 10.17487/RFC2822, April 2001,
<https://www.rfc-editor.org/info/rfc2822>.

[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<https://www.rfc-editor.org/info/rfc5322>.

[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
<https://www.rfc-editor.org/info/rfc5905>.

[TR35] Davis, M., Ed., "Unicode Technical Standard #35: Unicode
Locale Data Markup Language (LDML)",
<https://www.unicode.org/reports/
tr35/#UnicodeCalendarIdentifier>.

[TZDB] IANA, "Time zone and daylight saving time data",
<https://data.iana.org/time-zones/tz-link.html>.

[TZDB-NAMING]
IANA, "Theory and pragmatics of the tz code and data",
<https://data.iana.org/time-zones/theory.html>.

Acknowledgements

This specification benefits from work prepared by ECMA TC39,
specifically in the Temporal proposal.

Richard Gibson and Justin Grant provided editorial improvements. The
SEDATE WG Chairs Mark McFadden and Bron Gondwana, the latter also in
his role as CALEXT WG Chair, helped set up the structures needed to
navigate the multi-SDO environment. John Klensin critically
accompanied the development of this specification, which led to
significant improvements. The authors would also like to especially
thank Francesca Palombini for her AD review and for her overall
guidance during the development process.

Contributors

Justin Grant
Email: [email protected]

Authors' Addresses

Ujjwal Sharma
Igalia, S.L.
Bugallal Marchesi, 22, 1º
15008 A Coruña
Spain
Email: [email protected]

Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen
Germany
Phone: +49-421-218-63921
Email: [email protected]