reports/rfc/draft-newman-datetime-00.txt

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   5 Network Working Group                                          C. Newman
   6 Internet Draft: Date and Time on the Internet                   Innosoft
   7 Document: draft-newman-datetime-00.txt                     December 1996
   8
   9
  10                      Date and Time on the Internet
  11
  12
  13 Status of this memo
  14
  15      This document is an Internet Draft.  Internet Drafts are working
  16      documents of the Internet Engineering Task Force (IETF), its Areas,
  17      and its Working Groups.  Note that other groups may also distribute
  18      working documents as Internet Drafts.
  19
  20      Internet Drafts are draft documents valid for a maximum of six
  21      months.  Internet Drafts may be updated, replaced, or obsoleted by
  22      other documents at any time.  It is not appropriate to use Internet
  23      Drafts as reference material or to cite them other than as a
  24      ``working draft'' or ``work in progress``.
  25
  26      To learn the current status of any Internet-Draft, please check the
  27      1id-abstracts.txt listing contained in the Internet-Drafts Shadow
  28      Directories on ds.internic.net, nic.nordu.net, ftp.isi.edu, or
  29      munnari.oz.au.
  30
  31      A revised version of this draft document will be submitted to the
  32      IESG as a Proposed Standard for the Internet Community.  Discussion
  33      and suggestions for improvement are requested.  This document will
  34      expire six months after publication.  Distribution of this draft is
  35      unlimited.
  36
  37
  38
  39 1. Introduction
  40
  41      Date and time formats cause a lot of confusion and interoperability
  42      problems on the Internet.  This document will address many of the
  43      problems encountered and make recommendations to improve
  44      consistancy and interoperability when representing and using date
  45      and time in Internet protocols.
  46
  47      This document includes an Internet profile of the ISO 8601
  48      [ISO8601] standard for representation of dates and times.
  49
  50      [More detail work is needed, but I wanted to get this out before I
  51      go on vacation to see if it meets the basic requirements coming
  52      from the ASID and CALSCH working groups.  Places needing work are
  53
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  56 Newman                                                          [Page 1]
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  58 Internet Draft               Date and Time                 December 1996
  59
  60
  61      marked with XXX]
  62
  63
  64 2. Definitions
  65
  66      UTC  Coordinated Universal Time as maintained by the Bureau
  67           Internaational de l'Heure (International Time Bureau).
  68
  69      [XXX definitely need more definitions here.  It would be nice to
  70      reference a good time standard to define seconds, leap years, etc.]
  71
  72 3. Two or Three Digit Years
  73
  74      Two digit years are expected to cause great expense to many as the
  75      year 2000 approaches.  Many existing computer programs simply add
  76      or subtract 1900 from a two digit year.  Such programs will clearly
  77      stop functioning on the year 2000 and will have to be upgraded,
  78      possibly at great expense [XXX - ref to Wall Street Journal article
  79      on IRS year 2000 problems would be cool].  The following
  80      requirements are made of Internet protocols to address this
  81      problem:
  82
  83      o  Internet Protocols MUST generate four digit years in dates.
  84
  85      o  If a two digit year is received, the values 00-49 SHOULD be
  86      interpreted as referring to the 21st century (add 2000) and the
  87      values 50-99 SHOULD be interpreted as referring to the 20th century
  88      (add 1900).
  89
  90      o  Three digit years MUST be interpreted by adding 1900.
  91
  92
  93 4. Local Time
  94
  95 4.1. Coordinated Universal Time (UTC)
  96
  97      Because the daylight rules for local timezones are so convoluted
  98      [XXX-ref], true interoperability is best achieved by using
  99      Coordinated Universal Time (UTC) [XXX-ref].
 100
 101
 102 4.2. Local Offsets
 103
 104      The offset between local time and UTC is often useful information.
 105      For example, in electronic mail [IMAIL] the local offset provides a
 106      useful heuristic to determine the probability of a prompt response.
 107      Attempts to label local offsets with alphabetic strings have met
 108      with poor interoperability results in the past [IMAIL], [HOST-REQ].
 109
 110
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 112 Newman                                                          [Page 2]
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 116
 117      Therefore numeric offsets are now REQUIRED.  When the local offset
 118      is unknown, the offset "-00:00" MAY be used to indicate that the
 119      time is in UTC and the local offset is unknown.
 120
 121
 122 4.3. Unqualified Local Time
 123
 124      A number of devices currently connected to the Internet run their
 125      internal clocks in local time and are unaware of UTC.  While the
 126      Internet does have a tradition of accepting reality when creating
 127      specifications, this should not be done at the expense of
 128      interoperability.  Since interpretation of an unqualified local
 129      timezone will fail in approximately 23/24 of the globe, the
 130      interoperability problems of unqualified local time are deemed
 131      unacceptable for the Internet.  Devices which are unaware of the
 132      time in UTC MUST use one of the following techniques when
 133      communicating on the Internet:
 134
 135      o  Use Network Time Protocol [NTP] to obtain the time in UTC.
 136
 137      o  Use another host in the same local timezone as a gateway to the
 138      Internet.  This host MUST correct unqualified local times before
 139      they are transmitted to other hosts.
 140
 141      o  Prompt the user for the local timezone if it is aware of the
 142      daylight rules.  One technique to do this is by having the user
 143      select a major city in their timezone.  An alternative would be to
 144      show a list of the timezone labels defined in [section XXX].
 145
 146
 147 5. Date and Time formats
 148
 149      The date and time format defined in [IMAIL] and as amended by
 150      [HOST-REQ] may be referred to as "the Internet Mail Date/Time
 151      Format".  The profile of ISO 8601 defined in this section may be
 152      referred to as "the Internet Date/Time Format".  The following
 153      sections describe useful properties of a date and time format for
 154      interchange on the Internet.
 155
 156
 157 5.1. Ordering
 158
 159      If date and time components are ordered from least precise to most
 160      precise, then a useful property is achieved.  Assuming that the
 161      timezones of the dates and times are the same (e.g. all in UTC),
 162      then the date and time strings may be sorted as strings (e.g. using
 163      the strcmp() function in C) and a time-ordered sequence will
 164      result.  The presence of optional punctuation would violate this
 165
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 172
 173      characteristic.
 174
 175
 176 5.2. Human Readability
 177
 178      Human readability has proved to be a valuable feature of Internet
 179      protocols.  Human readable protocols greatly reduce the costs of
 180      debugging since telnet often suffices as a test client and network
 181      analysers need not be modified with knowledge of the protocol.  On
 182      the other hand, human readability sometimes results in
 183      interoperability problems.  For example, the date format
 184      "10/11/1996" is completely unsuitable for global interchange
 185      because it is interpreted differently in different countries.  In
 186      addition, the date format in [IMAIL] has resulted in
 187      interoperability problems when people assumed it was simply a text
 188      string and translated the three letter abbreviations to other
 189      languages or substituted date formats which were easier to generate
 190      (e.g. the format used by the C function ctime).  For this reason, a
 191      balance must be struck between human readability and
 192      interoperability.
 193
 194      Because no date and time format is readable according to the
 195      conventions of all countries, Internet clients SHOULD be prepared
 196      to transform dates into a display format suitable for the locality.
 197      This includes translating UTC to local time.
 198
 199
 200 5.3. Simplicity
 201
 202      The complete set of date and time formats specified in ISO 8601
 203      [ISO8601] is quite complex in an attempt to provide multiple
 204      representations and partial representations.  Appendix A contains
 205      an attempt to translate the complete syntax of ISO 8601 into ABNF
 206      as defined in [IMAIL].  Internet protocols have somewhat different
 207      requirements and simplicity has proved to be an important
 208      characteristic.  In addition, Internet protocols usually need
 209      complete specification of data in order to achieve true
 210      interoperability.  Therefore, the complete grammar for ISO 8601 is
 211      deemed too complex for most Internet protocols.
 212
 213      The following section defines an profile of ISO 8601 for use on the
 214      Internet.  It is a conformant subset of the ISO 8601 extended
 215      format.  Simplicity is achieved by making most fields and
 216      punctuation mandatory.
 217
 218
 219 5.4. Internet Date/Time Format
 220
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 224 Newman                                                          [Page 4]
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 228
 229      The following profile of ISO 8601 [ISO8601] dates SHOULD be used in
 230      new protocols on the Internet.  This is specified using ABNF as
 231      defined in [IMAIL].
 232
 233      date-fullyear   = 4DIGIT
 234      date-month      = 2DIGIT  ; 01-12
 235      date-mday       = 2DIGIT  ; 01-28, 01-29, 01-30, 01-31 based on month/year
 236      time-hour       = 2DIGIT  ; 00-24
 237      time-minute     = 2DIGIT  ; 00-59
 238      time-second     = 2DIGIT  ; 00-60
 239      time-secfrac    = "," 1*DIGIT
 240      time-numzone    = ("+" / "-") time-hour ":" time-minute
 241      time-zone       = "Z" / time-numzone
 242
 243      full-date       = date-fullyear "-" date-month "-" date-mday
 244      full-time       = time-hour ":" time-minute ":" time-second
 245                        [time-secfrac] time-zone
 246
 247      date-time       = full-date "T" full-time
 248
 249
 250 5.5 Examples
 251
 252      Here are two examples of this date and time format.
 253
 254      1985-04-12T23:20:50,5Z
 255
 256      This represents 20 minutes and 50.5 seconds after 11 PM on April
 257      12th, 1985 in UTC.
 258
 259      1996-12-19T16:39:57-08:00
 260
 261      This represents 39 minutes and 57 seconds after 4 PM on December
 262      19th, 1996 with an offset of -08:00 from UTC (Pacific Standard
 263      Time).
 264
 265
 266 6. IANA Registry of Timezone Names
 267
 268      [XXX - put good stuff here]
 269
 270
 271 7. References
 272
 273 [ISO8601] "Data elements and interchange formats -- Information
 274 interchange -- Representation of dates and times", ISO 8601:1988(E),
 275 International Organization for Standardization, June, 1988.
 276
 277
 278
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 284
 285 [IMAIL] Crocker, D., "Standard for the Format of Arpa Internet Text
 286  Messages", RFC 822, University of Delaware, August 1982.
 287
 288     <ftp://ds.internic.net/rfc/rfc822.txt>
 289
 290 [HOST-REQ] Braden, R., "Requirements for Internet Hosts -- Application
 291  and Support", RFC 1123, Internet Engineering Task Force, October 1989.
 292
 293     <ftp://ds.internic.net/rfc/rfc1123.txt>
 294
 295 [NTP] Mills, D., "Network Time Protocol version 2 specification and
 296  implementation", RFC 1119, September 1989.
 297
 298     <ftp://ds.internic.net/rfc/rfc1119.ps>
 299
 300
 301 8. Security Considerations
 302
 303      Since the local time zone of a site may be useful for determining a
 304      time when systems are less likely to be monitored and might be more
 305      susceptible to a security probe, some sites may wish to emit times
 306      in UTC only.  Others might consider this to be loss of useful
 307      functionality at the hands of paranoia.
 308
 309
 310 9. Author's Address
 311
 312 Chris Newman
 313 Innosoft International, Inc.
 314 1050 East Garvey Ave. South
 315 West Covina, CA 91790 USA
 316
 317 Email: chris.newman@innosoft.com
 318
 319 APPENDIX
 320
 321 A. ISO 8601 Collected ABNF
 322
 323      ISO 8601 does not specify a formal grammar for the date and time
 324      formats it defines.  The following is an attempt to create a formal
 325      grammar from ISO 8601.  This is informational only and may contain
 326      errors.  ISO 8601 remains the authoratative reference for the
 327      complete syntax.
 328
 329      date-century      = 2DIGIT  ; 00-99
 330      date-decade       =  DIGIT  ; 0-9
 331      date-subdecade    =  DIGIT  ; 0-9
 332      date-year         = date-decade date-subdecade
 333
 334
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 340
 341      date-fullyear     = date-century date-year
 342      date-month        = 2DIGIT  ; 01-12
 343      date-wday         =  DIGIT  ; 1-7  ; 1 is Monday, 7 is Sunday
 344      date-mday         = 2DIGIT  ; 01-28, 01-29, 01-30, 01-31 based on month/year
 345      date-yday         = 3DIGIT  ; 001-365, 001-366 based on year
 346      date-week         = 2DIGIT  ; 01-52, 01-53 based on year
 347
 348      datepart-fullyear = [date-century] date-year ["-"]
 349      datepart-ptyear   = "-" [date-subdecade ["-"]]
 350      datepart-wkyear   = datepart-ptyear / datepart-fullyear
 351
 352      dateopt-century   = "-" / date-century
 353      dateopt-fullyear  = "-" / datepart-fullyear
 354      dateopt-year      = "-" / (date-year ["-"])
 355      dateopt-month     = "-" / (date-month ["-"])
 356      dateopt-week      = "-" / (date-week ["-"])
 357
 358      datespec-full     = datepart-fullyear date-month ["-"] date-mday
 359      datespec-year     = date-century / dateopt-century date-year
 360      datespec-month    = "-" dateopt-year date-month [["-"] date-mday]
 361      datespec-mday     = "--" dateopt-month date-mday
 362      datespec-week     = datepart-wkyear "W" (date-week / dateopt-week date-wday)
 363      datespec-wday     = "---" date-wday
 364      datespec-yday     = dateopt-fullyear date-yday
 365
 366      date              = datespec-full / datespec-year / datespec-month /
 367           datespec-mday / datespec-week / datespec-wday / datespec-yday
 368
 369      Time:
 370
 371      time-hour         = 2DIGIT ; 00-24
 372      time-minute       = 2DIGIT ; 00-59
 373      time-second       = 2DIGIT ; 00-60
 374      time-fraction     = ("," / ".") 1*DIGIT
 375      time-numzone      = ("+" / "-") time-hour [[":"] time-minute]
 376      time-zone         = "Z" / time-numzone
 377
 378      timeopt-hour      = "-" / (time-hour [":"])
 379      timeopt-minute    = "-" / (time-minute [":"])
 380
 381      timespec-hour     = time-hour [[":"] time-minute [[":"] time-second]]
 382      timespec-minute   = timeopt-hour time-minute [[":"] time-second]
 383      timespec-second   = "-" timeopt-minute time-second
 384      timespec-base     = timespec-hour / timespec-minute / timespec-second
 385
 386      time              = timespec-base [time-fraction] [time-zone]
 387
 388      iso-date-time     = date "T" time
 389
 390
 391
 392 Newman                                                          [Page 7]
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 395
 396
 397      Durations (periods):
 398
 399      dur-second        = 1*DIGIT "S" dur-minute        = 1*DIGIT "M"
 400      [dur-second] dur-hour          = 1*DIGIT "H" [dur-minute] dur-time
 401      = "T" (dur-hour / dur-minute / dur-second) dur-day           =
 402      1*DIGIT "D" dur-week          = 1*DIGIT "W" dur-month         =
 403      1*DIGIT "M" [dur-day] dur-year          = 1*DIGIT "Y" [dur-month]
 404      dur-date          = (dur-day / dur-month / dur-year) [dur-time]
 405
 406      duration          = "P" (dur-date / dur-time / dur-week)
 407
 408      Periods:
 409
 410      period-explicit   = date-time "/" date-time period-start      =
 411      date-time "/" duration period-end        = duration "/" date-time
 412
 413      period            = period-explicit / period-start / period-end
 414
 415
 416 B. Zeller's Congruence [XXX-ref]
 417
 418      The following is sample C code which may be used to obtain the day
 419      of the week:
 420
 421      char *dayofweek[] = {
 422          "Sunday", "Monday", "Tuesday", "Wednesday",
 423          "Thursday", "Friday", "Saturday"
 424      };
 425
 426      void main()
 427      {
 428          int cent, year, day, month;
 429
 430          printf("Enter the year (4 digits): ");
 431          scanf("%d", &year);
 432          printf("\nEnter the month (1-12): ");
 433          scanf("%d", &month);
 434          printf("\nEnter the day of the month (1-31): ");
 435          scanf("%d", &day);
 436          month -= 2;
 437          if (month < 1) {
 438              month += 12;
 439              year--;
 440          }
 441          cent = year / 100;
 442          year %= 100;
 443          printf("The day of the week is: %s\n",
 444                 dayofweek[((26 * month - 2) / 10 + day + year
 445
 446
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 452
 453                            + year / 4 + cent / 4 - 2 * cent) % 7]);
 454      }
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 504 Newman                                                          [Page 9]
 505 \f