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Versions: 00 01                                                         
Network Working Group                                          C. Newman
Internet Draft: Date and Time on the Internet                   Innosoft
Document: draft-newman-datetime-00.txt                     December 1996


                     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
     munnari.oz.au.

     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
     unlimited.



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.

     [More detail work is needed, but I wanted to get this out before I
     go on vacation to see if it meets the basic requirements coming
     from the ASID and CALSCH working groups.  Places needing work are



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     marked with XXX]


2. Definitions

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

     [XXX definitely need more definitions here.  It would be nice to
     reference a good time standard to define seconds, leap years, etc.]

3. Two or Three 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 Wall Street Journal 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 SHOULD be
     interpreted as referring to the 21st century (add 2000) and the
     values 50-99 SHOULD be interpreted as referring to the 20th century
     (add 1900).

     o  Three digit years MUST be interpreted by adding 1900.


4. Local Time

4.1. Coordinated Universal Time (UTC)

     Because the daylight rules for local timezones are so convoluted
     [XXX-ref], true interoperability is best achieved by using
     Coordinated Universal Time (UTC) [XXX-ref].


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 met
     with poor interoperability results in the past [IMAIL], [HOST-REQ].



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     Therefore numeric offsets are now REQUIRED.  When the local offset
     is unknown, the offset "-00:00" MAY be used to indicate that the
     time is in UTC and the local offset is unknown.


4.3. 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
     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 if it is aware of the
     daylight rules.  One technique to do this is by having the user
     select a major city in their timezone.  An alternative would be to
     show a list of the timezone labels defined in [section XXX].


5. Date and Time formats

     The date and time format defined in [IMAIL] and as amended by
     [HOST-REQ] may be referred to as "the Internet Mail Date/Time
     Format".  The profile of ISO 8601 defined in this section may be
     referred to as "the Internet Date/Time Format".  The following
     sections describe useful properties of a date and time format for
     interchange on the Internet.


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



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     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
     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
     interoperability problems when people assumed it was simply a text
     string 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 includes translating UTC to local time.


5.3. 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
     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 an 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.


5.4. Internet Date/Time Format




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     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  ; 00-24
     time-minute     = 2DIGIT  ; 00-59
     time-second     = 2DIGIT  ; 00-60
     time-secfrac    = "," 1*DIGIT
     time-numzone    = ("+" / "-") time-hour ":" time-minute
     time-zone       = "Z" / time-numzone

     full-date       = date-fullyear "-" date-month "-" date-mday
     full-time       = time-hour ":" time-minute ":" time-second
                       [time-secfrac] time-zone

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


5.5 Examples

     Here are two examples of this date and time format.

     1985-04-12T23:20:50,5Z

     This represents 20 minutes and 50.5 seconds after 11 PM on April
     12th, 1985 in UTC.

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

     This represents 39 minutes and 57 seconds after 4 PM on December
     19th, 1996 with an offset of -08:00 from UTC (Pacific Standard
     Time).


6. IANA Registry of Timezone Names

     [XXX - put good stuff here]


7. References

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




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

    <ftp://ds.internic.net/rfc/rfc822.txt>

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

    <ftp://ds.internic.net/rfc/rfc1123.txt>

[NTP] Mills, D., "Network Time Protocol version 2 specification and
 implementation", RFC 1119, September 1989.

    <ftp://ds.internic.net/rfc/rfc1119.ps>


8. 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.


9. Author's Address

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

Email: chris.newman@innosoft.com

APPENDIX

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 for the
     complete syntax.

     date-century      = 2DIGIT  ; 00-99
     date-decade       =  DIGIT  ; 0-9
     date-subdecade    =  DIGIT  ; 0-9
     date-year         = date-decade date-subdecade



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     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:

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

     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



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     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)

     Periods:

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

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


B. Zeller's Congruence [XXX-ref]

     The following is sample C code which may be used to obtain the day
     of the week:

     char *dayofweek[] = {
         "Sunday", "Monday", "Tuesday", "Wednesday",
         "Thursday", "Friday", "Saturday"
     };

     void main()
     {
         int cent, year, day, month;

         printf("Enter the year (4 digits): ");
         scanf("%d", &year);
         printf("\nEnter the month (1-12): ");
         scanf("%d", &month);
         printf("\nEnter the day of the month (1-31): ");
         scanf("%d", &day);
         month -= 2;
         if (month < 1) {
             month += 12;
             year--;
         }
         cent = year / 100;
         year %= 100;
         printf("The day of the week is: %s\n",
                dayofweek[((26 * month - 2) / 10 + day + year



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                           + year / 4 + cent / 4 - 2 * cent) % 7]);
     }

















































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