Date and Time on the Internet: Timestamps with additional information
draft-ietf-sedate-datetime-extended-06

Serialising Extended Data About Times and Events               U. Sharma
Internet-Draft                                              Igalia, S.L.
Updates: 3339 (if approved)                                   C. Bormann
Intended status: Standards Track                  Universität Bremen TZI
Expires: 20 March 2023                                 16 September 2022

 Date and Time on the Internet: Timestamps with additional information
                 draft-ietf-sedate-datetime-extended-06

Abstract

   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.

   // The present version (-06) reflects the discussions at IETF 114.
   // In particular, RFC 3339 is now updated with respect to the
   // semantics of time zone offset Z.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-sedate-datetime-
   extended/.

   Discussion of this document takes place on the Serialising Extended
   Data About Times and Events (SEDATE) Working Group mailing list
   (mailto:sedate@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/sedate/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/sedate/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-sedate/draft-ietf-sedate-datetime-
   extended.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Updating RFC 3339 . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Internet Extended Date/Time format (IXDTF)  . . . . . . . . .   7
     3.1.  Informative . . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Registered  . . . . . . . . . . . . . . . . . . . . . . .   8
     3.3.  Optionally Critical . . . . . . . . . . . . . . . . . . .   8
     3.4.  Inconsistent time-offset/Time-Zone Information  . . . . .  10
   4.  Syntax Extensions to RFC 3339 . . . . . . . . . . . . . . . .  11
     4.1.  ABNF  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     4.2.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  12
   5.  The u-ca Suffix Key: Calendar Awareness . . . . . . . . . . .  13
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  16
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  16

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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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, and [ISO8601:1988] standardizes a
   widely-adopted timestamp format, which forms the basis of the
   Internet Date/Time Format [RFC3339].  However, this format only
   allows timestamps to contain very little additional relevant
   information, which means that, 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 already a pressing issue for applications that handle each
   instant with an associated time zone name, to take into account
   events such as daylight saving time transitions.  Most 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.

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 any 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:

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   *  Future time given as a local time in some specified time zone,
      where changes to the definition of that time zone (e.g., a
      political decision to enact or rescind daylight saving time)
      affect the instant in time corresponding with 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 TAI.

   However, additional information augmenting a fixed timestamp may be
   sufficient to detect an inconsistency between intention and the
   actual information in the timestamp, e.g., between the UTC offset and
   time zone name in the timestamp.  For instance, such an inconsistency
   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 the applications producing and consuming such a
      timestamp.

   While the information available is not generally sufficient to
   resolve the 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).

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      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 some 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

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      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].  Special IANA time
      zones such as Etc/GMT+10 can be used to represent timestamps
      outside country boundaries, e.g. a buoy in the middle of the
      Pacific Ocean (note that the Etc/GMT+10 time zone has a constant
      UTC Offset of -10:00 [sic!]).

      Note that 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, i.e., the additional information conveyed
      by using that time zone name is to change with the changed rules
      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.  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 annotation 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.  Updating RFC 3339

   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".

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   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].  The latter
   convention is in actual use, while the former always was handicapped
   by the fact that [ISO8601:1988] does not actually allow -00:00.

   Implementations that needed to express the semantics of -00:00
   therefore tended to use Z as a "neutral" offset instead.

   This specification updates RFC3339, aligning it with the actual
   practice of interpreting the local offset Z: this is no longer
   understood to "imply that UTC is the preferred reference point for
   the specified time".

   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 [ISO8601:1988] 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.  For the intents and
   purposes of the present specification, the local offset Z can 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 such a format that extends
   [RFC3339] for use in Internet protocols.

3.1.  Informative

   The format is intended to allow implementations to specify additional
   important information in addition to the bare 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.

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   When a suffix contains a repeated key or otherwise conflicting tags,
   implementations MUST give precedence to whichever value is positioned
   first.
   // I'd rather place a MUST NOT for this case, first.  This definitely
   // needs to be expanded into some general text about error handling.
   //
   // -- --- cabo

3.2.  Registered

   Actual 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.  Optionally Critical

   For the format defined here, suffix tags are always _optional_: They
   can be added or left out as desired by the generator of the string in
   Internet Extended Date/Time Format (IXDTF).  (An application might
   require the presence of specific suffix tags, though.)

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   Without further indication, they are also _elective_: Even if
   included in the IXDTF string, the recipient is free to ignore the
   suffix tag.  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).

   IXDTF strings such as:

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

   are internally inconsistent (see Section 3.4), as 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

   Similar with:

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

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

      |  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.  With the knowledge of how time zone offsets
      |  applied to Europe/Paris in the summer of 2022, it is equivalent
      |  to:
      |  
      |     2022-07-08T02:14:07+02:00[Europe/Paris]

   In contrast to this elective use of a suffix tag,

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   2022-07-08T00:14:07+01:00[!Europe/Paris]
   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 the
   IXDTF string as erroneous.

   Note that this does not mean that an application is disallowed to
   perform additional processing on inconsistent or unrecognized
   elective suffix tags, e.g., asking the user how to resolve the
   inconsistency.  It means it is not required to do so with elective
   suffix tags, but is required to reject or perform some other error
   handling when encountering inconsistent or unrecognized suffix tags
   marked as critical.

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]

                  Figure 1: Inconsistent IXDTF timestamps

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   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.  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 base their local offset and local time
   calculations on the time zone suffix given, i.e., using the Europe/
   London time zone rules.

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

               Figure 2: No inconsistency in IXDTF timestamps

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], ALPHA and DIGIT from Appendix B.1 of [RFC5234].

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   time-zone-initial = ALPHA / "." / "_"
   time-zone-char    = time-zone-initial / DIGIT / "-" / "+"
   time-zone-part    = time-zone-initial *13(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.

4.2.  Examples

   Here are 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 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: Adding a time zone name

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   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: Projecting to the Hebrew calendar

   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: 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 as defined by the [CLDR] data for [TR35].  At the
   time of writing, this information is collected in [CLDR-CALENDAR].

6.  IANA Considerations

   // RFC Editor: please replace RFCthis with the RFC number of this RFC
   // and remove this note.

   IANA is requested to establish a registry called "Timestamp Suffix
   Tag Keys".  Each entry in the registry shall consist of the
   information described in Section 3.2.  Initial contents of the
   registry are specified in Table 1.

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   +============+==============+==============+============+===========+
   | Key        | Registration | Description: | Change     |Reference  |
   | Identifier | status       |              | controller |           |
   +============+==============+==============+============+===========+
   | u-ca       | Permanent    | Preferred    | IESG       |Section 5  |
   |            |              | Calendar for |            |of RFCthis |
   |            |              | Presentation |            |           |
   +------------+--------------+--------------+------------+-----------+

       Table 1: Initial Content of Timestamp Suffix Tag Keys registry

   The registration policy [RFC8126] is "Specification Required" for
   permanent entries, and "Expert Review" for provisional ones.  In the
   second case, the expert is instructed to ascertain that a basic
   specification does exist, even if not complete or published yet.

7.  Security Considerations

   TBD

8.  References

8.1.  Normative References

   [BCP175]   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]   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>.

   [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>.

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   [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>.

   [RFC8126]  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>.

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

   [CLDR-CALENDAR]
              "cldr/common/bcp47/calendar.xml", <https://github.com/
              unicode-org/cldr/blob/main/common/bcp47/calendar.xml>.

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

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

   [JAVAZDT]  "Java SE 8, java.time.format, 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>.

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   [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>.

   [TR35]     "Unicode Technical Standard", <https://unicode-
              org.github.io/cldr/ldml/
              tr35-dates.html#Supplemental_Calendar_Data>.

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

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

Acknowledgements

   Richard Gibson provided some editorial improvements.

Contributors

   Justin Grant
   Email: justingrant.ietf.public@gmail.com

Authors' Addresses

   Ujjwal Sharma
   Igalia, S.L.
   Bugallal Marchesi, 22, 1º
   15008 A Coruña
   Spain
   Email: ryzokuken@igalia.com

   Carsten Bormann
   Universität Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany
   Phone: +49-421-218-63921
   Email: cabo@tzi.org

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