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Network Working Group                                          C. Newman
Internet Draft: Date and Time on the Internet                   Innosoft
Document: draft-newman-datetime-01.txt                      January 1997

                     Date and Time on the Internet

Status of this memo

     This document is an Internet Draft.  Internet Drafts are working
     documents of the Internet Engineering Task Force (IETF), its Areas,
     and its Working Groups.  Note that other groups may also distribute
     working documents as Internet Drafts.

     Internet Drafts are draft documents valid for a maximum of six
     months.  Internet Drafts may be updated, replaced, or obsoleted by
     other documents at any time.  It is not appropriate to use Internet
     Drafts as reference material or to cite them other than as a
     ``working draft'' or ``work in progress``.

     To learn the current status of any Internet-Draft, please check the
     1id-abstracts.txt listing contained in the Internet-Drafts Shadow
     Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or

     A revised version of this draft document will be submitted to the
     IESG as a Proposed Standard for the Internet Community.  Discussion
     and suggestions for improvement are requested.  This document will
     expire six months after publication.  Distribution of this draft is

1. Introduction

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

     This document includes an Internet profile of the ISO 8601
     [ISO8601] standard for representation of dates and times using the
     Gregorian calendar.

     Changes from draft -00:
      * added some more definitions and references (fixed NTP reference)

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      * language clarifications
      * include rules for day of month and leap seconds
      * disallow hour of 24
      * add registry of named timezones and local time format
      * use . instead of , for fractions of second
      * removed references to AM/PM
      * simplified appendix B program
      * added appendix C program to calculate leap year

     Controversial in last draft, but unchanged:
      * use of "T" as date/time separator.  Proposal is to use " " instead,
        but my reading of ISO 8601 does not permit that.
      * suggestion to make minutes offset from UTC optional.
      * interpretation of 2 digit years

     Open issues:
      * See controversial issues above.  More comment is welcome.
      * Are more definitions needed?
      * A number of the timezones in the initial registry list are
        duplicates for future times.  I already removed the Indiana county
        ones since they all duplicate Indianapolis for future times.
      * Need reference to good article demonstrating year 2000 problems.
      * Need commentary on registry and to set up address for registry.
      * Will add appendix D, E with sample POSIX generation/parsing code
        for section 5.6.  Markus Kuhn is working on code.

2. Definitions

     UTC         Coordinated Universal Time as maintained by the Bureau
                 Internaational de l'Heure (International Time Bureau).

     second      A basic unit of measurement of time in the
                 International System of Units.

     minute      A period of time of 60 seconds.

     hour        A period of time of 60 minutes.

     day         A period of time of 24 hours.

     leap year   In the 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 it shall be divisible by four hundred
                 an integral number of times.

     ABNF        Augmented Backus-Naur Form, a format used to represent

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                 permissible strings in a protocol or language, as
                 defined in [IMAIL].

     Email Date/Time Format
                 The date/time format used by Internet Mail as defined
                 by RFC 822 [IMAIL] and amended by RFC 1123 [HOST-REQ].

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

     For more information about time scales, see Appendix E of [NTP],
     Section 3 of [ISO8601], and the appropriate ITU documents [ITU-R-

3. Two Digit Years

     Two digit years are expected to cause great expense to many as the
     year 2000 approaches.  Many existing computer programs simply add
     or subtract 1900 from a two digit year.  Such programs will clearly
     stop functioning on the year 2000 and will have to be upgraded,
     possibly at great expense [XXX - ref to article on IRS year 2000
     problems would be cool].  The following requirements are made of
     Internet protocols to address this problem:

     o  Internet Protocols MUST generate four digit years in dates.

     o  If a two digit year is received, the values 00-49 MUST be
        interpreted as referring to the 21st century (add 2000) and the
        values 50-99 MUST be interpreted as referring to the 20th
        century (add 1900).  While different interpretations may result
        in a few more years of 2-digit usability for some applications,
        it is believed that a single interpretation for two digit years
        in all Internet protocols will result in better
        interoperability.  In addition, it is reasonable to expect all
        Internet Protocols using 2 digit dates to be upgraded by the
        year 2050.

     o  It is possible that a program using two digit years will
        represent years after 1999 as three digits.  This occurs if the
        program simply subtracts 1900 from the year and doesn't check
        the number of digits.  Programs wishing to robustly deal with
        dates generated by such broken software may add 1900 to three
        digit years.

     o  It is possible that a program using two digit years will
        represent years after 1999 as ":0", ":1", ... ":9", ";0", ...
        This occurs if the program simply subtracts 1900 from the year

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        and adds the decade to the US-ASCII character zero. Programs
        wishing to robustly deal with dates generated by such broken
        software should detect non-numeric decades and interpret

     The problems with two digit years amply demonstrate why all dates
     and times used in Internet protocols MUST be fully qualified.

4. Local Time

4.1. Coordinated Universal Time (UTC)

     Because the daylight rules for local timezones are so convoluted
     and can change based on local law at unpredictable times, true
     interoperability is best achieved by using Coordinated Universal
     Time (UTC).

4.2. Local Offsets

     The offset between local time and UTC is often useful information.
     For example, in electronic mail [IMAIL] 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 [IMAIL], [HOST-REQ].
     Therefore numeric offsets are now REQUIRED in Internet Mail
     Date/Time Format.

     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 22:58:00Z.

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 semanticly from an offset of "Z" which implies that UTC is
     the preferred reference point for the specified time.  This
     convention MAY also be used in the Email Date/Time Format.

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

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     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
     timezone will fail in approximately 23/24 of the globe, the
     interoperability problems of unqualified local time are deemed
     unacceptable for the Internet.  Devices which are unaware of the
     time in UTC MUST use one of the following techniques when
     communicating on the Internet:

     o  Use Network Time Protocol [NTP] to obtain the time in UTC.

     o  Use another host in the same local timezone as a gateway to the
        Internet.  This host MUST correct unqualified local times before
        they are transmitted to other hosts.

     o  Prompt the user for the local timezone and daylight savings

5. Date and Time formats

     This section discusses desirable qualities of date and time formats
     and defines a profile of ISO 8601 for use in new Internet
     protocols.  Email Date/Time Format lacks many of these
     characteristics and its use in new protocols is discouraged.

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
     timezones of the dates and times are the same (e.g. all in UTC),
     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

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
     analysers 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 [IMAIL] has resulted in

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     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

     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.

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

     The format defined below includes only one rarely used option:
     fractions of a second.  It is expected that this will be used only
     by applications which require strict ordering of date/time stamps
     or which have an unusual precision requirement.

5.4. Redundant Information

     If a date/time format includes redundant information, that
     introduces the possibility that the redunant 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 B),
     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.  Appendix A contains
     an attempt to translate the complete syntax of ISO 8601 into ABNF
     as defined in [IMAIL].  Internet protocols have somewhat different
     requirements and simplicity has proved to be an important
     characteristic.  In addition, Internet protocols usually need

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     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 profile of ISO 8601 for use on the
     Internet.  It is a conformant subset of the ISO 8601 extended
     format.  Simplicity is achieved by making most fields and
     punctuation mandatory.

5.6. Internet Date/Time Format

     The following profile of ISO 8601 [ISO8601] dates SHOULD be used in
     new protocols on the Internet.  This is specified using ABNF as
     defined in [IMAIL].

     date-fullyear   = 4DIGIT
     date-month      = 2DIGIT  ; 01-12
     date-mday       = 2DIGIT  ; 01-28, 01-29, 01-30, 01-31 based on month/year
     time-hour       = 2DIGIT  ; 00-23
     time-minute     = 2DIGIT  ; 00-59
     time-second     = 2DIGIT  ; 00-59, 00-60 based on leap second rules
     time-secfrac    = "." 1*DIGIT
     time-numoffset  = ("+" / "-") time-hour ":" time-minute
     time-offset     = "Z" / time-numoffset

     partial-time    = time-hour ":" time-minute ":" time-second
     full-date       = date-fullyear "-" date-month "-" date-mday
     full-time       = partial-time time-offset

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

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:

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     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

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

     The grammar element time-second may have the value "60" at the end
     of June (XXXX-06-30T23:59:60) or December (XXXX-12-31T23:59:60).
     At all other times the maximum value of time-second is "59".

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

5.8. Examples

     Here are three examples of Internet date/time format.


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


     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.


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

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6. Future Events in Local Time

     Some applications (e.g. calendaring) require the representation of
     repeating or future dates in local time.  Because the conversion
     rules between UTC and local time may change by season and political
     whim, it is necessary to label the local time zone with a standard
     label so that if new conversion rules are issued the interpretation
     of the time relative to UTC can be corrected.  For this reason, an
     IANA registry of timezone names which may be used to represent
     future dates is necessary.

6.1. Problems Too Hard to Solve

     Since local timezone rules are set by local governments, the only
     authoratative reference for such rules is those governments, most
     of which do not currently provide their rules on line in a computer
     parsible format.  In addition, local timezones were historically
     set by cities and towns, so attempting to exhaustively enumerate
     all historical timezones for use in representing past dates is not
     practical.  Attempting to predict where new timezones will be
     created as a subset of the area covered by an old timezone is also
     a hopeless prospect.

     Therefore the only formal part of the registry will be names for a
     minimal set of modern timezones.  As a convenience, the registry
     will also include the base UTC offset and daylight savings rules
     (if determinable) at the time of registration.  Because the UTC
     offset and rules may changed by other bodies, they will not be
     considered an authoratative part of the registry.

6.2. Prior Art

     An informal collection of timezone information is currently being
     maintained by volunteer Internet participants.  The current
     location of this information is:


     This is valuable work, and is used in some operating systems.  The
     initial set of timezone names for the IANA registry is a subset of
     the names collected by this effort.

6.3. Legal Characters in Timezone Names

     Only the US-ASCII characters A-Z, a-z, 0-9, "-", "_" and "/" are

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     legal in timezone names.  The basic format is the name of a
     continent or ocean followed by a "/" followed by the name of a city
     or political entity.  A political entity may be followed by a "/"
     and a subentity if necessary.  Timezone names SHOULD use the
     standard case in the registry, but MUST be interpreted in a case
     insensitive manner.   New timezone names SHOULD use "-" rather than
     "_", as the latter is difficult to see in some output contexts.

6.4. Template for IANA Registration of Timezone Names

     To: timezone@XXX
     Subject: Timezone Name Registration

     Timezone Name:

     ISO 3166 2-character country code:


6.5. Proceedure for IANA Registration of Timezone Names

     The IESG is responsible for appointing a reviewer of Timezone
     Names.  The job of this reviewer is to verify that the new timezone
     name has unique UTC rules, is likely to be used, fits the rules in
     section 6.3 and does not conflict unnecessarily with prior art
     (especially that mentioned in section 6.2).  Within two weeks of
     posting, the reviewer must take one of the following actions:

     (1) Pass the registration proposal to IANA.

     (2) Reject the registration proposal.

     (3) Recommend alterations to the registration proposal likely to make
         it acceptable.

     In order to assist the reviewer, the address timezone@XXX will be a
     public mailing list where registration proposals may be discussed.
     Subscription and unsubscription requests may be sent to timezone-

6.6. Initial List of IANA Timezone Names

     The following list will serve as the initial list of IANA Timezone
     Names.  This list was generated from the archive mentioned in
     section 6.2.  Some of these timezone names (especially within the

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     same country) are redundant for future dates, but compatibility
     with the timezone names in the databases discussed section 6.2. is

     Timezone Name                    Country
     -------------                    -------
     Europe/Andorra                     AD
     Asia/Dubai                         AE
     Asia/Kabul                         AF
     America/Antigua                    AG
     America/Anguilla                   AI
     Europe/Tirane                      AL
     Asia/Yerevan                       AM
     America/Curacao                    AN
     Africa/Luanda                      AO
     Antarctica/Casey                   AQ
     Antarctica/DumontDUrville          AQ
     Antarctica/Mawson                  AQ
     Antarctica/McMurdo                 AQ
     Antarctica/Palmer                  AQ
     Antarctica/South_Pole              AQ
     America/Buenos_Aires               AR
     America/Catamarca                  AR
     America/Cordoba                    AR
     America/Jujuy                      AR
     America/Mendoza                    AR
     America/Rosario                    AR
     Pacific/Pago_Pago                  AS
     Europe/Vienna                      AT
     Australia/Adelaide                 AU
     Australia/Brisbane                 AU
     Australia/Broken_Hill              AU
     Australia/Darwin                   AU
     Australia/Hobart                   AU
     Australia/Lindeman                 AU
     Australia/Lord_Howe                AU
     Australia/Melbourne                AU
     Australia/Perth                    AU
     Australia/Sydney                   AU
     America/Aruba                      AW
     Asia/Baku                          AZ
     Europe/Sarajevo                    BA
     America/Barbados                   BB
     Asia/Dacca                         BD
     Europe/Brussels                    BE
     Africa/Ouagadougou                 BF
     Europe/Sofia                       BG
     Asia/Bahrain                       BH

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     Africa/Bujumbura                   BI
     Africa/Porto-Novo                  BJ
     Atlantic/Bermuda                   BM
     Asia/Brunei                        BN
     America/La_Paz                     BO
     America/Cuiaba                     BR
     America/Fortaleza                  BR
     America/Maceio                     BR
     America/Manaus                     BR
     America/Noronha                    BR
     America/Porto_Acre                 BR
     America/Sao_Paulo                  BR
     America/Nassau                     BS
     Asia/Thimbu                        BT
     Africa/Gaborone                    BW
     Europe/Minsk                       BY
     America/Belize                     BZ
     America/Dawson                     CA
     America/Dawson_Creek               CA
     America/Edmonton                   CA
     America/Glace_Bay                  CA
     America/Goose_Bay                  CA
     America/Halifax                    CA
     America/Inuvik                     CA
     America/Iqaluit                    CA
     America/Montreal                   CA
     America/Nipigon                    CA
     America/Pangnirtung                CA
     America/Rainy_River                CA
     America/Rankin_Inlet               CA
     America/Regina                     CA
     America/St_Johns                   CA
     America/Swift_Current              CA
     America/Thunder_Bay                CA
     America/Vancouver                  CA
     America/Whitehorse                 CA
     America/Winnipeg                   CA
     America/Yellowknife                CA
     Indian/Cocos                       CC
     Africa/Bangui                      CF
     Africa/Brazzaville                 CG
     Europe/Zurich                      CH
     Africa/Abidjan                     CI
     Pacific/Rarotonga                  CK
     America/Santiago                   CL
     Pacific/Easter                     CL
     Africa/Douala                      CM
     Asia/Chungking                     CN

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     Asia/Harbin                        CN
     Asia/Kashgar                       CN
     Asia/Shanghai                      CN
     Asia/Urumqi                        CN
     America/Bogota                     CO
     America/Costa_Rica                 CR
     America/Havana                     CU
     Atlantic/Cape_Verde                CV
     Indian/Christmas                   CX
     Asia/Nicosia                       CY
     Europe/Prague                      CZ
     Europe/Berlin                      DE
     Africa/Djibouti                    DJ
     Europe/Copenhagen                  DK
     America/Dominica                   DM
     America/Santo_Domingo              DO
     Africa/Algiers                     DZ
     America/Guayaquil                  EC
     Pacific/Galapagos                  EC
     Europe/Tallinn                     EE
     Africa/Cairo                       EG
     Africa/El_Aaiun                    EH
     Africa/Asmera                      ER
     Africa/Ceuta                       ES
     Atlantic/Canary                    ES
     Europe/Madrid                      ES
     Africa/Addis_Ababa                 ET
     Europe/Helsinki                    FI
     Pacific/Fiji                       FJ
     Atlantic/Stanley                   FK
     Pacific/Kosrae                     FM
     Pacific/Ponape                     FM
     Pacific/Truk                       FM
     Pacific/Yap                        FM
     Atlantic/Faeroe                    FO
     Europe/Paris                       FR
     Africa/Libreville                  GA
     Europe/Belfast                     GB
     Europe/London                      GB
     America/Grenada                    GD
     Asia/Tbilisi                       GE
     America/Cayenne                    GF
     Africa/Accra                       GH
     Europe/Gibraltar                   GI
     America/Godthab                    GL
     America/Scoresbysund               GL
     America/Thule                      GL
     Africa/Banjul                      GM

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     Africa/Conakry                     GN
     America/Guadeloupe                 GP
     Africa/Malabo                      GQ
     Europe/Athens                      GR
     Atlantic/South_Georgia             GS
     America/Guatemala                  GT
     Pacific/Guam                       GU
     Africa/Bissau                      GW
     America/Guyana                     GY
     Asia/Hong_Kong                     HK
     America/Tegucigalpa                HN
     Europe/Zagreb                      HR
     America/Port-au-Prince             HT
     Europe/Budapest                    HU
     Asia/Jakarta                       ID
     Asia/Jayapura                      ID
     Asia/Ujung_Pandang                 ID
     Europe/Dublin                      IE
     Asia/Gaza                          IL
     Asia/Jerusalem                     IL
     Asia/Calcutta                      IN
     Indian/Chagos                      IO
     Asia/Baghdad                       IQ
     Asia/Tehran                        IR
     Atlantic/Reykjavik                 IS
     Europe/Rome                        IT
     America/Jamaica                    JM
     Asia/Amman                         JO
     Asia/Ishigaki                      JP
     Asia/Tokyo                         JP
     Africa/Nairobi                     KE
     Asia/Bishkek                       KG
     Asia/Phnom_Penh                    KH
     Pacific/Enderbury                  KI
     Pacific/Kiritimati                 KI
     Pacific/Tarawa                     KI
     Indian/Comoro                      KM
     America/St_Kitts                   KN
     Asia/Pyongyang                     KP
     Asia/Seoul                         KR
     Asia/Kuwait                        KW
     America/Cayman                     KY
     Asia/Alma-Ata                      KZ
     Asia/Aqtau                         KZ
     Asia/Aqtobe                        KZ
     Asia/Vientiane                     LA
     Asia/Beirut                        LB
     America/St_Lucia                   LC

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     Europe/Vaduz                       LI
     Asia/Colombo                       LK
     Africa/Monrovia                    LR
     Africa/Maseru                      LS
     Europe/Vilnius                     LT
     Europe/Luxembourg                  LU
     Europe/Riga                        LV
     Africa/Tripoli                     LY
     Africa/Casablanca                  MA
     Europe/Monaco                      MC
     Europe/Chisinau                    MD
     Indian/Antananarivo                MG
     Pacific/Kwajalein                  MH
     Pacific/Majuro                     MH
     Europe/Skopje                      MK
     Africa/Bamako                      ML
     Africa/Timbuktu                    ML
     Asia/Rangoon                       MM
     Asia/Ulan_Bator                    MN
     Asia/Macao                         MO
     Pacific/Saipan                     MP
     America/Martinique                 MQ
     Africa/Nouakchott                  MR
     America/Montserrat                 MS
     Europe/Malta                       MT
     Indian/Mauritius                   MU
     Indian/Maldives                    MV
     Africa/Blantyre                    MW
     America/Ensenada                   MX
     America/Mazatlan                   MX
     America/Mexico_City                MX
     America/Tijuana                    MX
     Asia/Kuala_Lumpur                  MY
     Asia/Kuching                       MY
     Africa/Maputo                      MZ
     Africa/Windhoek                    NA
     Pacific/Noumea                     NC
     Africa/Niamey                      NE
     Pacific/Norfolk                    NF
     Africa/Lagos                       NG
     America/Managua                    NI
     Europe/Amsterdam                   NL
     Europe/Oslo                        NO
     Asia/Katmandu                      NP
     Pacific/Nauru                      NR
     Pacific/Niue                       NU
     Pacific/Auckland                   NZ
     Pacific/Chatham                    NZ

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     Asia/Muscat                        OM
     America/Panama                     PA
     America/Lima                       PE
     Pacific/Gambier                    PF
     Pacific/Marquesas                  PF
     Pacific/Tahiti                     PF
     Pacific/Port_Moresby               PG
     Asia/Manila                        PH
     Asia/Karachi                       PK
     Europe/Warsaw                      PL
     America/Miquelon                   PM
     Pacific/Pitcairn                   PN
     America/Puerto_Rico                PR
     Atlantic/Azores                    PT
     Atlantic/Madeira                   PT
     Europe/Lisbon                      PT
     Pacific/Palau                      PW
     America/Asuncion                   PY
     Asia/Qatar                         QA
     Indian/Reunion                     RE
     Europe/Bucharest                   RO
     Asia/Anadyr                        RU
     Asia/Irkutsk                       RU
     Asia/Kamchatka                     RU
     Asia/Krasnoyarsk                   RU
     Asia/Magadan                       RU
     Asia/Novosibirsk                   RU
     Asia/Omsk                          RU
     Asia/Vladivostok                   RU
     Asia/Yakutsk                       RU
     Asia/Yekaterinburg                 RU
     Europe/Kaliningrad                 RU
     Europe/Moscow                      RU
     Europe/Samara                      RU
     Africa/Kigali                      RW
     Asia/Riyadh                        SA
     Pacific/Guadalcanal                SB
     Indian/Mahe                        SC
     Africa/Khartoum                    SD
     Europe/Stockholm                   SE
     Asia/Singapore                     SG
     Atlantic/St_Helena                 SH
     Europe/Ljubljana                   SI
     Arctic/Longyearbyen                SJ
     Atlantic/Jan_Mayen                 SJ
     Europe/Bratislava                  SK
     Africa/Freetown                    SL
     Europe/San_Marino                  SM

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     Africa/Dakar                       SN
     Africa/Mogadishu                   SO
     America/Paramaribo                 SR
     Africa/Sao_Tome                    ST
     America/El_Salvador                SV
     Asia/Damascus                      SY
     Africa/Mbabane                     SZ
     America/Grand_Turk                 TC
     Africa/Ndjamena                    TD
     Indian/Kerguelen                   TF
     Africa/Lome                        TG
     Asia/Bangkok                       TH
     Asia/Dushanbe                      TJ
     Pacific/Fakaofo                    TK
     Asia/Ashkhabad                     TM
     Africa/Tunis                       TN
     Pacific/Tongatapu                  TO
     Europe/Istanbul                    TR
     America/Port_of_Spain              TT
     Pacific/Funafuti                   TV
     Asia/Taipei                        TW
     Africa/Dar_es_Salaam               TZ
     Europe/Kiev                        UA
     Europe/Simferopol                  UA
     Africa/Kampala                     UG
     Pacific/Johnston                   UM
     Pacific/Midway                     UM
     Pacific/Wake                       UM
     America/Adak                       US
     America/Anchorage                  US
     America/Boise                      US
     America/Chicago                    US
     America/Denver                     US
     America/Detroit                    US
     America/Indianapolis               US
     America/Juneau                     US
     America/Los_Angeles                US
     America/Louisville                 US
     America/Menominee                  US
     America/New_York                   US
     America/Nome                       US
     America/Phoenix                    US
     America/Shiprock                   US
     America/Yakutat                    US
     Pacific/Honolulu                   US
     America/Montevideo                 UY
     Asia/Tashkent                      UZ
     Europe/Vatican                     VA

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     America/St_Vincent                 VC
     America/Caracas                    VE
     America/Tortola                    VG
     America/St_Thomas                  VI
     Asia/Saigon                        VN
     Pacific/Efate                      VU
     Pacific/Wallis                     WF
     Pacific/Apia                       WS
     Asia/Aden                          YE
     Indian/Mayotte                     YT
     Europe/Belgrade                    YU
     Africa/Johannesburg                ZA
     Africa/Lusaka                      ZM
     Africa/Kinshasa                    ZR
     Africa/Lubumbashi                  ZR
     Africa/Harare                      ZW

6.7. Local Date/Time Format

     The following format MAY be used to refer to future dates in a
     local timezone.  This is defined based on the format in section

     zone-char = ALPHA / DIGIT / "-" / "_" / "/"
     zone-name = 1*zone_char
                    ; case insensitive interpretation
     offset-hint = time-numoffset

     local-datetime = full-date "T" partial-time " " zone-name
                      [" " offset-hint]

     A local-datetime represents an event relative to a specific local
     timezone.  The offset-hint represents the generator's prediction of
     what the UTC offset will be at that local time, and may become
     incorrect if the rules for the specified zone are changed.  The
     offset-hint MAY be omitted if the generating program only knows
     local time, but the zone-name is REQUIRED.  This format SHOULD NOT
     be used for timestamps or past events.

6.8. Examples

     Here are some examples of Local Date/Time Format:

     1999-12-31T23:59:59 America/New_York -05:00

     This represents a time one (or two if there's a leap second) second

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     before the year 2000 in the timezone used in New York City in North
     America (currently U.S. Eastern Time).  The offset-hint is the
     number to add to the local time to get an estimate UTC for that
     date, so this will probably be equivalent to 2000-01-01T04:59:59Z.

     2000-12-31T23:59:59 Australia/Adelaide +09:30

     This represents a time one (or two if there's a leap second) second
     before the 21st century in Adelaide, Australia.  The hint suggests
     that this will be equivalent to 2000-12-31T14:29:59Z.

     2000-03-31T02:00:00 America/Los_Angeles -08:00

     The represents a time of the 2nd hour on the 31st of March in Los
     Angeles, USA.  The hint suggests that would be equivalent to 2000-
     03-31T10:00:00Z.  However, if the U.S. government were to adopt the
     daylight savings rules currently used by the European Union, which
     change daylight savings on the last Sunday of March, then the time
     would be equivalent to 2000-03-31T09:00:00Z.

7. Acknowledgements

     May thanks to the following people who have provided helpful advice
     for this document: Ned Freed, Neal McBurnett, David Keegel, Markus
     Kuhn, Paul Eggert and Robert Elz.  Thanks are also due to
     participants of the IETF Calendaring/Scheduling working group
     mailing list, and participants of the timezone mailing list.

8. References

[Zeller] Chr. Zeller, "Kalender-Formeln", Acta Mathematica, Vol. 9,
 Nov 1886.

[IMAIL] Crocker, D., "Standard for the Format of Arpa Internet Text
 Messages", RFC 822, University of Delaware, August 1982.


[ISO8601] "Data elements and interchange formats -- Information
 interchange -- Representation of dates and times", ISO 8601:1988(E),
 International Organization for Standardization, June, 1988.

[HOST-REQ] Braden, R., "Requirements for Internet Hosts -- Application
 and Support", RFC 1123, Internet Engineering Task Force, October 1989.


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[NTP] Mills, D., "Network Time Protocol (Version 3) Specification,
 Implementation and Analysis", RFC 1305, University of Delaware, March


[ITU-R-TF] International Telecommunication Union Recommendations for
 Time Signals and Frequency Standards Emissions.


9. 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.

10. Author's Address

Chris Newman
Innosoft International, Inc.
1050 East Garvey Ave. South
West Covina, CA 91790 USA

Email: chris.newman@innosoft.com


A. ISO 8601 Collected ABNF

     ISO 8601 does not specify a formal grammar for the date and time
     formats it defines.  The following is an attempt to create a formal
     grammar from ISO 8601.  This is informational only and may contain
     errors.  ISO 8601 remains the authoratative reference.

     Note that due to ambiguities in ISO 8601, some interpretations had
     to be made.  First, ISO 8601 is not clear if mixtures of basic and
     extended format are permissible.  This grammar permits mixtures.
     ISO 8601 is not clear on whether an hour of 24 is permissible only
     if minutes and seconds are 0.  This assumes that an hour of 24 is
     permissible in any context.  Restrictions on date-mday in section
     5.7 apply.  ISO 8601 states that the "T" may be omitted under some
     circumstances.  This grammar requires the "T" to avoid ambiguity.

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     ISO 8601 also requires (in section that a decimal fraction
     be proceeded by a "0" if less than unity.  Annex B.2 of ISO 8601
     gives examples where the decimal fractions are not preceeded by a
     "0".  This grammar assumes section is correct and that
     Annex B.2 is in error.

     date-century    = 2DIGIT  ; 00-99
     date-decade     =  DIGIT  ; 0-9
     date-subdecade  =  DIGIT  ; 0-9
     date-year       = date-decade date-subdecade
     date-fullyear   = date-century date-year
     date-month      = 2DIGIT  ; 01-12
     date-wday       =  DIGIT  ; 1-7  ; 1 is Monday, 7 is Sunday
     date-mday       = 2DIGIT  ; 01-28, 01-29, 01-30, 01-31 based on month/year
     date-yday       = 3DIGIT  ; 001-365, 001-366 based on year
     date-week       = 2DIGIT  ; 01-52, 01-53 based on year

     datepart-fullyear = [date-century] date-year ["-"]
     datepart-ptyear   = "-" [date-subdecade ["-"]]
     datepart-wkyear   = datepart-ptyear / datepart-fullyear

     dateopt-century   = "-" / date-century
     dateopt-fullyear  = "-" / datepart-fullyear
     dateopt-year      = "-" / (date-year ["-"])
     dateopt-month     = "-" / (date-month ["-"])
     dateopt-week      = "-" / (date-week ["-"])

     datespec-full     = datepart-fullyear date-month ["-"] date-mday
     datespec-year     = date-century / dateopt-century date-year
     datespec-month    = "-" dateopt-year date-month [["-"] date-mday]
     datespec-mday     = "--" dateopt-month date-mday
     datespec-week     = datepart-wkyear "W"
                         (date-week / dateopt-week date-wday)
     datespec-wday     = "---" date-wday
     datespec-yday     = dateopt-fullyear date-yday

     date              = datespec-full / datespec-year / datespec-month /
         datespec-mday / datespec-week / datespec-wday / datespec-yday


     time-hour         = 2DIGIT ; 00-24
     time-minute       = 2DIGIT ; 00-59
     time-second       = 2DIGIT ; 00-59, 00-60 based on leap-second rules
     time-fraction     = ("," / ".") 1*DIGIT
     time-numoffset    = ("+" / "-") time-hour [[":"] time-minute]
     time-zone         = "Z" / time-numoffset

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     timeopt-hour      = "-" / (time-hour [":"])
     timeopt-minute    = "-" / (time-minute [":"])

     timespec-hour     = time-hour [[":"] time-minute [[":"] time-second]]
     timespec-minute   = timeopt-hour time-minute [[":"] time-second]
     timespec-second   = "-" timeopt-minute time-second
     timespec-base     = timespec-hour / timespec-minute / timespec-second

     time              = timespec-base [time-fraction] [time-zone]

     iso-date-time     = date "T" time

     Durations (periods):

     dur-second        = 1*DIGIT "S"
     dur-minute        = 1*DIGIT "M" [dur-second]
     dur-hour          = 1*DIGIT "H" [dur-minute]
     dur-time          = "T" (dur-hour / dur-minute / dur-second)
     dur-day           = 1*DIGIT "D"
     dur-week          = 1*DIGIT "W"
     dur-month         = 1*DIGIT "M" [dur-day]
     dur-year          = 1*DIGIT "Y" [dur-month]
     dur-date          = (dur-day / dur-month / dur-year) [dur-time]

     duration          = "P" (dur-date / dur-time / dur-week)


     period-explicit   = date-time "/" date-time
     period-start      = date-time "/" duration
     period-end        = duration "/" date-time

     period            = period-explicit / period-start / period-end

B. Day of the Week

     The following is a sample C subroutine loosly based on Zeller's
     Congruence [Zeller] which may be used to obtain the day of the

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

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

C. Leap Years

     Here's 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));

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