Calendaring Extensions Working Group                           U. Sharma
Internet-Draft                                              Igalia, S.L.
Obsoletes: 3339 (if approved)                            11 January 2021
Intended status: Standards Track
Expires: 15 July 2021


 Date and Time on the Internet: Timestamps with additional information
                  draft-ryzokuken-datetime-extended-00

Abstract

   This document defines a date and time format for use in Internet
   protocols for representation of dates and times using the proleptic
   Gregorian calendar, with optional extensions representing additional
   information including a time zone and a calendaring system.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 15 July 2021.

Copyright Notice

   Copyright (c) 2021 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.




Sharma                    Expires 15 July 2021                  [Page 1]


Internet-Draft             Timestamps Extended              January 2021


Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Two Digit Years . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Local Time  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Coordinated Universal Time (UTC)  . . . . . . . . . . . .   4
     4.2.  Local Offsets . . . . . . . . . . . . . . . . . . . . . .   5
     4.3.  Unknown Local Offset Convention . . . . . . . . . . . . .   5
     4.4.  Unqualified Local Time  . . . . . . . . . . . . . . . . .   5
   5.  Date and Time format  . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Ordering  . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Human Readability . . . . . . . . . . . . . . . . . . . .   6
     5.3.  Rarely Used Options . . . . . . . . . . . . . . . . . . .   7
     5.4.  Redundant Information . . . . . . . . . . . . . . . . . .   7
     5.5.  Simplicity  . . . . . . . . . . . . . . . . . . . . . . .   7
     5.6.  Internet Date/Time Format . . . . . . . . . . . . . . . .   7
     5.7.  Restrictions  . . . . . . . . . . . . . . . . . . . . . .   9
     5.8.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   7.  Normative references  . . . . . . . . . . . . . . . . . . . .  12
   8.  Bibliography  . . . . . . . . . . . . . . . . . . . . . . . .  12
   Appendix A.  Day of the Week  . . . . . . . . . . . . . . . . . .  13
   Appendix B.  Leap Years . . . . . . . . . . . . . . . . . . . . .  13
   Appendix C.  Leap Seconds . . . . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   Date and time formats cause a lot of confusion and interoperability
   problems on the Internet.  This document addresses many of the
   problems encountered and makes recommendations to improve consistency
   and interoperability when representing and using date and time in
   Internet protocols.

   This document includes an extension to an Internet profile of the
   [ISO8601] standard for representation of dates and times using the
   proleptic Gregorian calendar alongside any additional information.
   Any dates and times that are required to be handled in a different
   calendar system need to include the ID of the calendar which can be
   handle by the implementations approximately.  In the absence of a
   calendar ID, the proleptic Gregorian calendar system is assumed.

   There are many ways in which date and time values might appear in
   Internet protocols: this document focuses on just one common usage,
   viz. timestamps for Internet protocol events.  This limited
   consideration has the following consequences:




Sharma                    Expires 15 July 2021                  [Page 2]


Internet-Draft             Timestamps Extended              January 2021


   *  All dates and times are assumed to be in the "current era",
      somewhere between 0000AD and 9999AD.

   *  All times expressed have a stated relationship (offset) to
      Coordinated Universal Time (UTC).  Certain applications require
      the presence of a time zone in order to perform scheduling as well
      as handle Daylight Savings Time transitions properly.  In that
      case, an optional time zone ID may be included.

   *  Timestamps can express times that occurred before the introduction
      of UTC.  Such timestamps are expressed relative to universal time,
      using the best available practice at the stated time.

   *  Date and time expressions indicate an instant in time.
      Description of time periods, or intervals, is not covered here.

2.  Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   UTC  Coordinated Universal Time as maintained by the Bureau
      International des Poids et Mesures (BIPM).

   second  A basic unit of measurement of time in the International
      System of Units.  It is defined as the duration of 9,192,631,770
      cycles of microwave light absorbed or emitted by the hyperfine
      transition of cesium-133 atoms in their ground state undisturbed
      by external fields.

   minute  A period of time of 60 seconds.  However, see also the
      restrictions in section Section 5.7 and Appendix C for how leap
      seconds are denoted within minutes.

   hour  A period of time of 60 minutes.

   day  A period of time of 24 hours.

   leap year  In the proleptic Gregorian calendar, a year which has 366
      days.  A leap year is a year whose number is divisible by four an
      integral number of times, except that if it is a centennial year
      (i.e. divisible by one hundred) it shall also be divisible by four
      hundred an integral number of times.

   ABNF  Augmented Backus-Naur Form, a format used to represent
      permissible strings in a protocol or language, as defined in
      [RFC2234].



Sharma                    Expires 15 July 2021                  [Page 3]


Internet-Draft             Timestamps Extended              January 2021


   Email Date/Time Format  The date/time format used by Internet Mail as
      defined by [RFC2822].

   Internet Date/Time Format  The date/time format defined in section 5
      of this document.

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

   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".

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

   Calendar  A calendar that is included in the Unicode CLDR data as a
      valid calendar for use in BCP47 language tags.

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

3.  Two Digit Years

   The use of 2 (and 3) digit years was allowed but deprecated in
   [RFC3339], the predecessor of this document.

   The use of such a format is no longer allowed, and implementations
   should use either a standard 4-digit year or the extended 6-digit
   value with a sign.

4.  Local Time

4.1.  Coordinated Universal Time (UTC)

   Because the daylight saving rules for local time zones are so
   convoluted and can change based on local law at unpredictable times,
   true interoperability is best achieved by using Coordinated Universal
   Time (UTC).  This specification by itself does not cater to local
   time zone rules.  However, certain implementations may be expected
   to.  For these situations, a timestamp may additionally include a
   local time zone that the implementations can take into account.









Sharma                    Expires 15 July 2021                  [Page 4]


Internet-Draft             Timestamps Extended              January 2021


4.2.  Local Offsets

   The offset between local time and UTC is often useful information.
   For example, in electronic mail (RFC2822, [RFC2822]) the local offset
   provides a useful heuristic to determine the probability of a prompt
   response.  Attempts to label local offsets with alphabetic strings
   have resulted in poor interoperability in the past [RFC1123].  As a
   result, RFC2822 [RFC2822] has made numeric offsets mandatory.

   Numeric offsets are calculated as "local time minus UTC".  So the
   equivalent time in UTC can be determined by subtracting the offset
   from the local time.  For example, "18:50:00-04:00" is the same time
   as "22:50:00Z".  (This example shows negative offsets handled by
   adding the absolute value of the offset.)

   Numeric offsets may differ from UTC by any number of seconds, or even
   a fraction of seconds.  This can be easily represented by including
   an optional seconds value in the offset, which may further optionally
   include a fraction of seconds behind a decimal point, for example
   "+12:34:56.789".  This is especially useful in the case of certain
   historical time zones.

4.3.  Unknown Local Offset Convention

   If the time in UTC is known, but the offset to local time is unknown,
   this can be represented with an offset of "-00:00".  This differs
   semantically from an offset of "Z" or "+00:00", which imply that UTC
   is the preferred reference point for the specified time.  RFC2822
   [RFC2822] describes a similar convention for email.

4.4.  Unqualified Local Time

   A number of devices currently connected to the Internet run their
   internal clocks in local time and are unaware of UTC.  While the
   Internet does have a tradition of accepting reality when creating
   specifications, this should not be done at the expense of
   interoperability.  Since interpretation of an unqualified local time
   zone will fail in approximately 23/24 of the globe, the
   interoperability problems of unqualified local time are deemed
   unacceptable for the Internet.  Systems that are configured with a
   local time, are unaware of the corresponding UTC offset, and depend
   on time synchronization with other Internet systems, MUST use a
   mechanism that ensures correct synchronization with UTC.  Some
   suitable mechanisms are:

   *  Use Network Time Protocol [RFC1305] to obtain the time in UTC.





Sharma                    Expires 15 July 2021                  [Page 5]


Internet-Draft             Timestamps Extended              January 2021


   *  Use another host in the same local time zone as a gateway to the
      Internet.  This host MUST correct unqualified local times that are
      transmitted to other hosts.

   *  Prompt the user for the local time zone and daylight saving rule
      settings.

5.  Date and Time format

   This section discusses desirable qualities of date and time formats
   and defines a format that extends the profile of ISO 8601 for use in
   Internet protocols.

5.1.  Ordering

   If date and time components are ordered from least precise to most
   precise, then a useful property is achieved.  Assuming that the time
   zones of the dates and times are the same (e.g., all in UTC),
   expressed using the same string (e.g., all "Z" or all "+00:00"), all
   times have the same number of fractional second digits, and they all
   have the same suffix (or none), then the date and time strings may be
   sorted as strings (e.g., using the "strcmp()" function in C) and a
   time-ordered sequence will result.  The presence of optional
   punctuation would violate this characteristic.

5.2.  Human Readability

   Human readability has proved to be a valuable feature of Internet
   protocols.  Human readable protocols greatly reduce the costs of
   debugging since telnet often suffices as a test client and network
   analyzers need not be modified with knowledge of the protocol.  On
   the other hand, human readability sometimes results in
   interoperability problems.  For example, the date format "10/11/1996"
   is completely unsuitable for global interchange because it is
   interpreted differently in different countries.  In addition, the
   date format in (RFC822) has resulted in interoperability problems
   when people assumed any text string was permitted and translated the
   three letter abbreviations to other languages or substituted date
   formats which were easier to generate (e.g. the format used by the C
   function "ctime").  For this reason, a balance must be struck between
   human readability and interoperability.

   Because no date and time format is readable according to the
   conventions of all countries, Internet clients SHOULD be prepared to
   transform dates into a display format suitable for the locality.
   This may include translating UTC to local time as well as converting
   from the Gregorian calendar to the viewer's preferred calendar.




Sharma                    Expires 15 July 2021                  [Page 6]


Internet-Draft             Timestamps Extended              January 2021


5.3.  Rarely Used Options

   A format which includes rarely used options is likely to cause
   interoperability problems.  This is because rarely used options are
   less likely to be used in alpha or beta testing, so bugs in parsing
   are less likely to be discovered.  Rarely used options should be made
   mandatory or omitted for the sake of interoperability whenever
   possible.

5.4.  Redundant Information

   If a date/time format includes redundant information, that introduces
   the possibility that the redundant information will not correlate.
   For example, including the day of the week in a date/time format
   introduces the possibility that the day of week is incorrect but the
   date is correct, or vice versa.  Since it is not difficult to compute
   the day of week from a date (see Appendix A), the day of week should
   not be included in a date/time format.

5.5.  Simplicity

   The complete set of date and time formats specified in ISO 8601
   [ISO8601] is quite complex in an attempt to provide multiple
   representations and partial representations.  Internet protocols have
   somewhat different requirements and simplicity has proved to be an
   important characteristic.  In addition, Internet protocols usually
   need complete specification of data in order to achieve true
   interoperability.  Therefore, the complete grammar for ISO 8601 is
   deemed too complex for most Internet protocols.

   The following section defines a format that in an extension of a
   profile of ISO 8601 for use on the Internet.  It is a conformant
   subset of the ISO 8601 extended format with additional information
   optionally suffixed.  Simplicity is achieved by making most fields
   and punctuation mandatory.

5.6.  Internet Date/Time Format

   The following extension of a profile of [ISO8601] dates SHOULD be
   used in new protocols on the Internet.  This is specified using the
   syntax description notation defined in [RFC2234].










Sharma                    Expires 15 July 2021                  [Page 7]


Internet-Draft             Timestamps Extended              January 2021


   date-year      = 4DIGIT / ("+" / "-") 6DIGIT
   date-month     = 2DIGIT ; 01-12
   date-mday      = 2DIGIT ; 01-28, 01-29, 01-30, 01-31 based on month/year
   date-full      = date-year "-" date-month "-" date-mday

   time-hour      = 2DIGIT ; 00-23
   time-minute    = 2DIGIT ; 00-59
   time-second    = 2DIGIT ; 00-58, 00-59, 00-60 based on leap second rules
   time-secfrac   = "." 1*DIGIT
   time-partial   = time-hour ":" time-minute ":" time-second [time-secfrac]
   time-numoffset = ("+" / "-") time-partial
   time-offset    = "Z" / time-numoffset
   time-full      = time-partial time-offset

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

   calendar-part  = 3*8(ALPHA / DIGIT)
   calendar-id    = calendar-part *("-" calendar-part)
   calendar       = "[c=" calendar-id "]"

   suffix-part    = "[" 1*ALPHA "=" 1*VCHAR "]"
   suffix         = [time-zone] [calendar] *suffix-part

   date-time      = date-full "T" time-full suffix

                                  Figure 1

      |  NOTE 1: Per [RFC2234] and ISO8601, the "T" and "Z" characters
      |  in this syntax may alternatively be lower case "t" or "z"
      |  respectively.

   This date/time format may be used in some environments or contexts
   that distinguish between the upper- and lower-case letters 'A'-'Z'
   and 'a'-'z' (e.g.  XML).  Specifications that use this format in such
   environments MAY further limit the date/time syntax so that the
   letters 'T' and 'Z' used in the date/time syntax must always be upper
   case.  Applications that generate this format SHOULD use upper case
   letters.

      |  NOTE 2: ISO 8601 defines date and time separated by "T".
      |  Applications using this syntax may choose, for the sake of
      |  readability, to specify a full-date and full-time separated by
      |  (say) a space character.





Sharma                    Expires 15 July 2021                  [Page 8]


Internet-Draft             Timestamps Extended              January 2021


5.7.  Restrictions

   The grammar element date-mday represents the day number within the
   current month.  The maximum value varies based on the month and year
   as follows:

    +==============+=====================+============================+
    | Month Number | Month/Year          | Maximum value of date-mday |
    +==============+=====================+============================+
    | 01           | January             | 31                         |
    +--------------+---------------------+----------------------------+
    | 02           | February, normal    | 28                         |
    +--------------+---------------------+----------------------------+
    | 02           | February, leap year | 29                         |
    +--------------+---------------------+----------------------------+
    | 03           | March               | 31                         |
    +--------------+---------------------+----------------------------+
    | 04           | April               | 30                         |
    +--------------+---------------------+----------------------------+
    | 05           | May                 | 31                         |
    +--------------+---------------------+----------------------------+
    | 06           | June                | 30                         |
    +--------------+---------------------+----------------------------+
    | 07           | July                | 31                         |
    +--------------+---------------------+----------------------------+
    | 08           | August              | 31                         |
    +--------------+---------------------+----------------------------+
    | 09           | September           | 30                         |
    +--------------+---------------------+----------------------------+
    | 10           | October             | 31                         |
    +--------------+---------------------+----------------------------+
    | 11           | November            | 30                         |
    +--------------+---------------------+----------------------------+
    | 12           | December            | 31                         |
    +--------------+---------------------+----------------------------+

                        Table 1: Days in each month

   Appendix B contains sample C code to determine if a year is a leap
   year.











Sharma                    Expires 15 July 2021                  [Page 9]


Internet-Draft             Timestamps Extended              January 2021


   The grammar element time-second may have the value "60" at the end of
   months in which a leap second occurs - to date: June (XXXX-06-
   30T23:59:60Z) or December (XXXX-12-31T23:59:60Z); see Appendix C for
   a table of leap seconds.  It is also possible for a leap second to be
   subtracted, at which times the maximum value of time-second is "58".
   At all other times the maximum value of time-second is "59".
   Further, in time zones other than "Z", the leap second point is
   shifted by the zone offset (so it happens at the same instant around
   the globe).

   Leap seconds cannot be predicted far into the future.  The
   International Earth Rotation Service publishes bulletins (IERS) that
   announce leap seconds with a few weeks' warning.  Applications should
   not generate timestamps involving inserted leap seconds until after
   the leap seconds are announced.

   Although ISO 8601 permits the hour to be "24", this extension of a
   profile of ISO 8601 only allows values between "00" and "23" for the
   hour in order to reduce confusion.

5.8.  Examples

   Here are some examples of Internet date/time format.

   1985-04-12T23:20:50.52Z

                                  Figure 2

   This represents 20 minutes and 50.52 seconds after the 23rd hour of
   April 12th, 1985 in UTC.

   +001985-04-12T23:20:50.52Z

                                  Figure 3

   This represents the same instant as the previous example but with the
   expanded 6-digit year format.

   1996-12-19T16:39:57-08:00

                                  Figure 4

   This represents 39 minutes and 57 seconds after the 16th hour of
   December 19th, 1996 with an offset of -08:00 from UTC (Pacific
   Standard Time).  Note that this is equivalent to 1996-12-20T00:39:57Z
   in UTC.

   1996-12-19T16:39:57-08:00[America/Los_Angeles]



Sharma                    Expires 15 July 2021                 [Page 10]


Internet-Draft             Timestamps Extended              January 2021


                                  Figure 5

   This represents the exact same instant as the previous example but
   additionally specifies the human time zone associated with it for
   time zone aware implementations to take into account.

   1990-12-31T23:59:60Z

                                  Figure 6

   This represents the leap second inserted at the end of 1990.

   1990-12-31T15:59:60-08:00

                                  Figure 7

   This represents the same leap second in Pacific Standard Time, 8
   hours behind UTC.

   1937-01-01T12:00:27.87+00:19:32.130

                                  Figure 8

   This represents the same instant of time as noon, January 1, 1937,
   Netherlands time.  Standard time in the Netherlands was exactly 19
   minutes and 32.13 seconds ahead of UTC by law from 1909-05-01 through
   1937-06-30.

   1937-01-01T12:00:27.87+00:19:32.130[c=japanese]

                                  Figure 9

   This represents the exact same instant as the previous example but
   additionally specifies the human calendar associated with it for
   calendar aware implementations to take into account.

   1937-01-01T12:00:27.87+00:19:32.130[foo=bar][baz=bat]

                                 Figure 10

   This timestamp utilizes the generalized format to declare two
   additional pieces of information in the suffix that can be
   interpreted by any compatible implementations and ignored otherwise.








Sharma                    Expires 15 July 2021                 [Page 11]


Internet-Draft             Timestamps Extended              January 2021


6.  Security Considerations

   Since the local time zone of a site may be useful for determining a
   time when systems are less likely to be monitored and might be more
   susceptible to a security probe, some sites may wish to emit times in
   UTC only.  Others might consider this to be loss of useful
   functionality at the hands of paranoia.

7.  Normative references

   [RFC2822]  Resnick, P., Ed., "Internet Message Format", IETF RFC
              2822, IETF RFC 2822, DOI 10.17487/RFC2822, April 2001,
              <https://www.rfc-editor.org/info/rfc2822>.

   [RFC2234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", IETF RFC 2234, IETF RFC 2234,
              DOI 10.17487/RFC2234, November 1997,
              <https://www.rfc-editor.org/info/rfc2234>.

   [RFC1123]  Braden, R., Ed., "Requirements for Internet
              Hosts — Application and Support", IETF RFC 1123, IETF RFC
              1123, DOI 10.17487/RFC1123, October 1989,
              <https://www.rfc-editor.org/info/rfc1123>.

   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
              Specification, Implementation and Analysis", IETF RFC
              1305, IETF RFC 1305, DOI 10.17487/RFC1305, March 1992,
              <https://www.rfc-editor.org/info/rfc1305>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", IETF RFC 2119, IETF RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

8.  Bibliography

   [ISO8601]  International Organization for Standardization, "Data
              elements and interchange formats", ISO 8601:1988, June
              1988, <https://www.iso.org/standard/15903.html>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", IETF RFC 3339, IETF RFC 3339,
              DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/info/rfc3339>.







Sharma                    Expires 15 July 2021                 [Page 12]


Internet-Draft             Timestamps Extended              January 2021


Appendix A.  Day of the Week

   The following is a sample C subroutine loosely based on Zeller's
   Congruence (ZELLER) which may be used to obtain the day of the week
   for dates on or after 0000-03-01:

   char *day_of_week(int day, int month, int year)
   {
       int cent;
       char *dayofweek[] = {
           "Sunday", "Monday", "Tuesday", "Wednesday",
           "Thursday", "Friday", "Saturday"
       };

       /* adjust months so February is the last one */
       month -= 2;
       if (month < 1) {
           month += 12;
           --year;
       }
       /* split by century */
       cent = year / 100;
       year %= 100;
       return (dayofweek[((26 * month - 2) / 10 + day + year
                       + year / 4 + cent / 4 + 5 * cent) % 7]);
   }

                                 Figure 11

Appendix B.  Leap Years

   Here is a sample C subroutine to calculate if a year is a leap year:

   /* This returns non-zero if year is a leap year.  Must use 4 digit
       year.
   */
   int leap_year(int year)
   {
       return (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0));
   }

                                 Figure 12

Appendix C.  Leap Seconds

   Information about leap seconds can be found at the US Navy
   Oceanography Portal (https://www.usno.navy.mil/USNO/time/master-
   clock/leap-seconds).  In particular, it notes that:



Sharma                    Expires 15 July 2021                 [Page 13]


Internet-Draft             Timestamps Extended              January 2021


   |  The decision to introduce a leap second in UTC is the
   |  responsibility of the International Earth Rotation Service (IERS).
   |  According to the CCIR Recommendation, first preference is given to
   |  the opportunities at the end of December and June, and second
   |  preference to those at the end of March and September.

   When required, insertion of a leap second occurs as an extra second
   at the end of a day in UTC, represented by a timestamp of the form
   YYYY-MM-DDT23:59:60Z.  A leap second occurs simultaneously in all
   time zones, so that time zone relationships are not affected.  See
   section Section 5.8 for some examples of leap second times.

   The following table is an excerpt from the table maintained by the
   United States Naval Observatory.  The source data is located at the
   US Navy Oceanography Portal (ftp://maia.usno.navy.mil/ser7/tai-
   utc.dat).

   This table shows the date of the leap second, and the difference
   between the time standard TAI (which isn't adjusted by leap seconds)
   and UTC after that leap second.

               +============+=============================+
               | UTC Date   | TAI - UTC After Leap Second |
               +============+=============================+
               | 1972-06-30 | 11                          |
               +------------+-----------------------------+
               | 1972-12-31 | 12                          |
               +------------+-----------------------------+
               | 1973-12-31 | 13                          |
               +------------+-----------------------------+
               | 1974-12-31 | 14                          |
               +------------+-----------------------------+
               | 1975-12-31 | 15                          |
               +------------+-----------------------------+
               | 1976-12-31 | 16                          |
               +------------+-----------------------------+
               | 1977-12-31 | 17                          |
               +------------+-----------------------------+
               | 1978-12-31 | 18                          |
               +------------+-----------------------------+
               | 1979-12-31 | 19                          |
               +------------+-----------------------------+
               | 1981-06-30 | 20                          |
               +------------+-----------------------------+
               | 1982-06-30 | 21                          |
               +------------+-----------------------------+
               | 1983-06-30 | 22                          |
               +------------+-----------------------------+



Sharma                    Expires 15 July 2021                 [Page 14]


Internet-Draft             Timestamps Extended              January 2021


               | 1985-06-30 | 23                          |
               +------------+-----------------------------+
               | 1987-12-31 | 24                          |
               +------------+-----------------------------+
               | 1989-12-31 | 25                          |
               +------------+-----------------------------+
               | 1990-12-31 | 26                          |
               +------------+-----------------------------+
               | 1992-06-30 | 27                          |
               +------------+-----------------------------+
               | 1993-06-30 | 28                          |
               +------------+-----------------------------+
               | 1994-06-30 | 29                          |
               +------------+-----------------------------+
               | 1995-12-31 | 30                          |
               +------------+-----------------------------+
               | 1997-06-30 | 31                          |
               +------------+-----------------------------+
               | 1998-12-31 | 32                          |
               +------------+-----------------------------+

                      Table 2: Historic leap seconds

Author's Address

   Ujjwal Sharma
   Igalia, S.L.

   Email: ryzokuken@igalia.com






















Sharma                    Expires 15 July 2021                 [Page 15]