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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
+   Provisions Relating to IETF Documents
+   (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
+   to this document.  Code Components extracted from this document must
+   include Simplified BSD License text as described in Section 4.e of
+   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