Concise Binary Object Representation (CBOR) Tags for Date
RFC 8943

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Internet Engineering Task Force (IETF)                          M. Jones
Request for Comments: 8943                                     Microsoft
Category: Standards Track                                     A. Nadalin
ISSN: 2070-1721                                              Independent
                                                              J. Richter
                                             pdv Financial Software GmbH
                                                           November 2020

       Concise Binary Object Representation (CBOR) Tags for Date

Abstract

   The Concise Binary Object Representation (CBOR), as specified in RFC
   7049, is a data format whose design goals include the possibility of
   extremely small code size, fairly small message size, and
   extensibility without the need for version negotiation.

   In CBOR, one point of extensibility is the definition of CBOR tags.
   RFC 7049 defines two tags for time: CBOR tag 0 (date/time string as
   per RFC 3339) and tag 1 (POSIX "seconds since the epoch").  Since
   then, additional requirements have become known.  This specification
   defines a CBOR tag for a date text string (as per RFC 3339) for
   applications needing a textual date representation within the
   Gregorian calendar without a time.  It also defines a CBOR tag for
   days since the date 1970-01-01 in the Gregorian calendar for
   applications needing a numeric date representation without a time.
   This specification is the reference document for IANA registration of
   the CBOR tags defined.

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/rfc8943.

Copyright Notice

   Copyright (c) 2020 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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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Calendar Dates
       1.1.1.  Example Date Representations
     1.2.  Comparing Dates
     1.3.  Comparing Dates and Date/Time Values
   2.  IANA Considerations
     2.1.  Concise Binary Object Representation (CBOR) Tags
           Registrations
   3.  Security Considerations
   4.  References
     4.1.  Normative References
     4.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The Concise Binary Object Representation (CBOR) [RFC7049] provides
   for the interchange of structured data without a requirement for a
   pre-agreed schema.  RFC 7049 defines a basic set of data types, as
   well as a tagging mechanism that enables extending the set of data
   types supported via an IANA registry.

   This specification defines a CBOR tag for a text string representing
   a date without a time.  The tagged text string is represented as
   specified by the RFC 3339 [RFC3339] "full-date" production.  Per RFC
   3339, this represents a date within the Gregorian calendar.

   This specification also defines a CBOR tag for an integer
   representing a date without a time.  The tagged integer is an
   unsigned or negative value indicating the number of days since the
   Gregorian calendar date 1970-01-01.  As an implementation note, this
   value has a constant offset from the Modified Julian Date value
   (which is defined by the Smithsonian Astrophysical Observatory as the
   number of days since November 17, 1858); this value is the Modified
   Julian Date minus 40587.

   Note that since both tags are for dates without times, times of day,
   time zones, and leap seconds are not applicable to these values.
   These tags are both for representations of Gregorian calendar dates.

1.1.  Calendar Dates

   Calendar dates are used for numerous human use cases, such as marking
   the dates of significant events.  For instance, John Lennon was born
   on October 9, 1940 and died on December 8, 1980.  One such use case
   is driver's licenses, which typically include a date of birth.  The
   dates used in this specification use the Gregorian calendar, as do
   those in RFC 3339 [RFC3339].  The time zones and actual times of
   these events are intentionally not represented in the calendar date.

   The epoch chosen for the second tag, which represents days since the
   Gregorian calendar date 1970-01-01, is related to the IEEE Std
   1003.1, 2013 Edition [POSIX.1] time epoch 1970-01-01T00:00:00Z UTC
   only insofar as both contain the date 1970-01-01.  This should not be
   construed as indicating that dates using this tag represent either a
   specific time of day and/or time zone.

   The day of the week (Sunday, Monday, Tuesday, etc.) is not explicitly
   represented in either of these date formats.  However, deterministic
   algorithms that are beyond the scope of this specification can be
   used to derive the day of the week in the Gregorian calendar from
   dates represented in both of these formats.

1.1.1.  Example Date Representations

   This table contains example representations for dates using both
   tags.

               +==================+==============+=========+
               | Date             | Tag 1004     | Tag 100 |
               +==================+==============+=========+
               | October 9, 1940  | "1940-10-09" | -10676  |
               +------------------+--------------+---------+
               | December 8, 1980 | "1980-12-08" | 3994    |
               +------------------+--------------+---------+

                                  Table 1

1.2.  Comparing Dates

   Comparison of dates in "full-date" format can be accomplished by
   normal string comparison, since, by design, the digits representing
   the date are in fixed format and ordered from most significant to
   least significant.  Comparison of numeric dates representing days
   since 1970-01-01 can be performed by normal integer comparison.
   Comparison of dates in other formats or using other calendars require
   conversions that are beyond the scope of this specification.

   Note that different dates may correspond to the same moment in time,
   depending upon the time zone in which the date was determined.  For
   instance, at many times of the day, a conference call occurring on a
   particular date in Japan will simultaneously occur on the previous
   date in Hawaii; at many times of the day, Japan's Friday corresponds
   with Hawaii's Thursday.

1.3.  Comparing Dates and Date/Time Values

   Comparing dates with date/time values, which represent a particular
   moment in time, is beyond the scope of this specification.  That
   said, if a date is augmented with a time zone and time of day, a
   specific date/time value can be determined, and comparing that date/
   time value to others becomes possible.  For instance, if one were to
   augment John Lennon's birth date of October 9, 1940 with the time of
   day and time zone of his birth, then it would be possible to derive a
   date/time at which he was born that could be compared with other
   date/time values.

2.  IANA Considerations

2.1.  Concise Binary Object Representation (CBOR) Tags Registrations

   This section registers the following values in the IANA "Concise
   Binary Object Representation (CBOR) Tags" registry [IANA.cbor-tags].

   Tag:  1004
   Data Item:  UTF-8 text string
   Semantics:  [RFC3339] full-date string
   Reference:  RFC 8943

   Tag:  100 (ASCII 'd')
   Data Item:  Unsigned or negative integer
   Semantics:  Number of days since the epoch date 1970-01-01
   Reference:  RFC 8943

3.  Security Considerations

   The security considerations of RFC 7049 apply; the tags introduced
   here are not expected to raise security considerations beyond those.

   A date, of course, has significant security considerations.  These
   include the exploitation of ambiguities where the date is security
   relevant or where the date is used in access control decisions.

   When using a calendar date for decision making (for example, access
   control), it needs to be noted that since calendar dates do not
   represent a specific point in time, the results of the evaluation can
   differ depending upon where the decision is made.  For instance, a
   person may have reached their 21st birthday in Japan while
   simultaneously being a day short of their 21st birthday in Hawaii.
   Similarly, it would be inappropriate to use only a date to trigger
   certificate expiration, since a date corresponds to a range of times
   worldwide rather than a specific point in time that is independent of
   geographic location.

4.  References

4.1.  Normative References

   [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>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

4.2.  Informative References

   [IANA.cbor-tags]
              IANA, "Concise Binary Object Representation (CBOR) Tags",
              <https://www.iana.org/assignments/cbor-tags>.

   [POSIX.1]  IEEE, "The Open Group Base Specifications Issue 7", 2013
              Edition, IEEE Std 1003.1, 2013,
              <https://pubs.opengroup.org/
              onlinepubs/9699919799.2013edition>.

   [TIME-TAGS]
              Bormann, C., Gamari, B., and H. Birkholz, "Concise Binary
              Object Representation (CBOR) Tags for Time, Duration, and
              Period", Work in Progress, Internet-Draft, draft-bormann-
              cbor-time-tag-03, 9 March 2020,
              <https://tools.ietf.org/html/draft-bormann-cbor-time-tag-
              03>.

Acknowledgements

   Thanks to Carsten Bormann for supporting creation of this
   specification.  Parts of the explanatory text in this specification
   come from [TIME-TAGS].

   Thanks to these people for reviews of the specification: Henk
   Birkholz, Carsten Bormann, Samita Chakrabarti, Roman Danyliw, Linda
   Dunbar, Benjamin Kaduk, Erik Kline, Warren Kumari, Barry Leiba,
   Thiago Macieira, Francesca Palombini, Michael Richardson, Kyle Rose,
   Jim Schaad, Juergen Schoenwaelder, Éric Vyncke, Robert Wilton, and
   Dale Worley.

Authors' Addresses

   Michael B. Jones
   Microsoft

   Email: mbj@microsoft.com
   URI:   https://self-issued.info/

   Anthony Nadalin
   Independent

   Email: nadalin@prodigy.net

   Jörg Richter
   pdv Financial Software GmbH

   Email: joerg.richter@pdv-fs.de