Internet-Draft Timestamps Extended December 2021
Sharma & Bormann Expires 6 June 2022 [Page]
Serialising Extended Data About Times and Events
Intended Status:
Standards Track
U. Sharma
Igalia, S.L.
C. Bormann
Universität Bremen TZI

Date and Time on the Internet: Timestamps with additional information


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

About This Document

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

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Table of Contents

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] standardizes a widely-adopted timestamp format, which forms the basis of [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 things like DST transitions. Most of these applications attach the timezone 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 it needs to be represented in.

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.

This document does not address extensions to the format where the semantic result no longer is 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 (e.g., a political decision to enact or rescind daylight savings time) changes the actual time in UTC time.
  • Time given as a "floating time", i.e., a local time without information as to which time zone this local time refers to.
  • The use of time scales different from UTC, such as TAI.

However, the additional information provided that augments a fixed timestamp may be sufficient to detect an inconsistency between intention and the actual information given in the timestamp, e.g., between the additional timezone information given and the timezone offset recorded in the timestamp. For instance, such an inconsistency might arise because of:

  • Political decisions as discussed above, 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.


Coordinated Universal Time, as maintained since 1988 by the Bureau International des Poids et Mesures (BIPM) in conjunction with leap seconds as announced by the International Earth Rotation and Reference Frames Service [IERS]. From 1972 through 1987 UTC was maintained entirely by Bureau International de l'Heure (BIH). Before 1972 UTC was not generally recognized and civil time was determined by individual jurisdictions using different techniques for attempting to follow Universal Time based on measuring the rotation of the earth.

UTC is often mistakenly referred to as GMT, an earlier timescale UTC was designed to be a useful successor for.


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 Date/Time Format:

The date/time format defined in section 3 of this document.


This term is used in this document to refer to an unambiguous representation of some instant in time.


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".

Time Zone:

A time zone that is included in the Time Zone Database (often called tz or zoneinfo) maintained by IANA.


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

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

2. Extended Date/Time format

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.

2.1. Informative

The format should 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 key of a tag can be split into two parts by including a hyphen/minus sign "-"; the first part (including the "-") can then be used as a namespace. The value of a tag can be a hyphen/minus delimited list of multiple values.

Out of these tags, applications can build an informative suffix at the end with as many tags as required.

Keys are case-sensitive. Values are case-sensitive unless otherwise specified.

In case a suffix repeats a key or otherwise contains 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

2.2. Namespaced

Suffix keys identify a namespace. By including a hyphen/minus sign "-", the namespace can be separated from the rest of the key; if no hyphen/minus sign is included, the whole key is the namespace.

For example, if "u-" is a namespace for the Unicode consortium, a calendar as defined by that consortium could be included as u-ca=<value>.

An IANA registry for namespaces can be used to allocate namespaces for specific applications, as defined in Section 4. Two namespaces are allocated by the present document:

  • "u-" for keys defined by the Unicode consortium.
  • "x-" for keys used within experiments. Such keys are not for general interchange and MUST be rejected by a recipient unless that is specifically enabled for an experiment. See [RFC6648] for additional considerations about "x-" namespaces.
  • In addition, for CLDR extensions: I don't know how this would be used, so I can't edit this text.--- cabo

    • There must be a namespace-key and it is restricted to 2 alphanum characters.
    • A suffix-value is limited to 3*8alphanum.

Additional namespaces can be registered under an Expert review policy, providing a description for the intended use. This may be a general concept, or a specific organization that is intended to register keys within this namespace.

2.3. Registered

Actual keys are registered by supplying the information in Figure 1:

Figure 1: Registration record for a tag key

'Identifier' contains the key name.

'Description' contains the name and description of the namespace.

'Comments' is an OPTIONAL field and MAY contain a broader description of the namespace.

'Added' contains the date the key's definition was published in the "date-full" format specified in Figure 2. For example: 2004-06-28 represents June 28, 2004, in the Gregorian calendar.

'RFC' contains the RFC number assigned to the namespace.

'Authority' contains the name of the maintaining authority for the namespace.

'Contact_Email' contains the email address used to contact the maintaining authority.

'Mailing_List' contains the URL or subscription email address of the mailing list used by the maintaining authority.

'URL' contains the URL of the registry for this namespace.

3. Syntax Extensions to RFC 3339

3.1. ABNF

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

The extended date/time format is described by the rule date-time-ext. date-time is imported from Section 5.6 of [RFC3339], ALPHA and DIGIT from Appendix B.1 of [RFC5234].

time-zone-initial = ALPHA / "." / "_"
time-zone-char    = time-zone-initial / DIGIT / "-" / "+"
time-zone-part    = time-zone-initial *13(time-zone-char)
                    ; but not "." or ".."
time-zone-name    = time-zone-part *("/" time-zone-part)
time-zone         = "[" time-zone-name "]"

namespace         = 1*alphanum
namespace-key     = 1*alphanum
suffix-key        = namespace ["-" namespace-key]

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

date-time-ext     = date-time suffix

alphanum          = ALPHA / DIGIT
Figure 2: ABNF grammar of extensions to RFC 3339

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

3.2. Examples

Here are some examples of Internet extended date/time format.

Figure 3 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.

Figure 4: Adding a timezone name

Figure 4 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.

Figure 5: Projecting to the Hebrew calendar

Figure 5 represents the exact same instant, but it informs calendar-aware implementations that they should project it to the Hebrew calendar.

Figure 6: Adding tags in private use namespaces

Figure 6, based on Figure 3, utilizes the private use namespace to declare two additional pieces of information in the suffix that can be interpreted by any compatible implementations and ignored otherwise.

4. IANA Considerations

Define a registry that can contain both namespaces and keys. Namespaces can be recognized by ending with a hyphen/minus. Actual keys do not. See Section 2.3 for the detailed information (to be edited).

The policy is "RFC required", "Specification Required", ???We need to define the policy for both namespaces and full keys.--- cabo [RFC8126].

5. Security Considerations


6. References

6.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Klyne, G. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, DOI 10.17487/RFC3339, , <>.
Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, , <>.
Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.

6.2. Informative References

"Unicode CLDR Project", <>.
"International Earth Rotation Service Bulletins", <>.
ISO, "Data elements and interchange formats — Information interchange — Representation of dates and times", ISO 8601:1988, , <>.
"ITU-R TF.460-6. Standard-frequency and time-signal emissions", , <>.
"Java SE 8, java.time.format, DateTimeFormatter: ISO_ZONED_DATE_TIME", <>.
Mills, D., "Network Time Protocol (Version 3) Specification, Implementation and Analysis", RFC 1305, DOI 10.17487/RFC1305, , <>.
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, , <>.

Appendix A. Acknowledgements


Authors' Addresses

Ujjwal Sharma
Igalia, S.L.
Bugallal Marchesi, 22, 1º
15008 A Coruña
Carsten Bormann
Universität Bremen TZI
Postfach 330440
D-28359 Bremen