



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.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

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: 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
