DEC Answers Leap Year Complaint

      ******DEC INTERNAL USE ONLY******
      
      SPR NUMBER:                  11-60903
      
      ANSWER CATEGORY:             UE
      MAINTENANCE HOURS:           1
      DUPLICATE PROBLEM:           N
      DUPLICATE SPR NUMBER(S):
      
      OPERATING SYSTEM:            VAX/VMS
      
      O.S. VERSION:                V3.2
      PRODUCT:                     VAX/VMS
      PRODUCT VERSION:             V3.2
      COMPONENT:                   Run-Time Library
      SUB-COMPONENT:               LIB$ routines
      
      
      DATE ANSWERED:               13-Oct-1983
      
      MAINTAINER:                  Stanley Rabinowitz
      
      
      ATTACHMENT:                  N
      
      PUBLICATION INSTRUCTIONS:    N
      
      SPR PROBLEM ABSTRACT:        User claims year 2000 should not be a leap year.
      
      TITLE:                       -
      PUBLICATIONS:                -
      ADDITIONAL O.S. VERSIONS:
      ADDITIONAL PRODUCT VERSIONS:
      COMPONENT SEQUENCE NUMBER:
      
      SUPERSEDES:
      
      TYPE OF ARTICLE:
      
                                  ANSWER CATEGORIES
      
      CG=1=CORRECTION GIVEN       RS=5=RESTRICTION              SG=9=SUGGESTION
      FN=2=FIXED IN NEXT RELEASE  CS=6=CUSTOMER SUPPORTED       IQ=10=INQUIRY
      DE=3=DOCUMENTATION ERROR    NR=7=NON-REPRODUCIBLE         HW=11=HARDWARE
      UE=4=USER ERROR             II=8=INSUFFICIENT INFORMATION
      
                                  TYPE OF ARTICLE
      
      F=OPTIONAL FEATURE PATCH    N=NOTE
      M=MANDATORY PATCH           R=RESTRICTION
      
                               FOR MAINTENANCE USE
      
                           ******END OF DEC USE ONLY******
                                  D I G I T A L
      
                                 SPR ANSWER FORM
      
      SPR NO. 11-60903
      
      
                 SYSTEM   VERSION   PRODUCT   VERSION   COMPONENT
      SOFTWARE:  VAX/VMS  V3.2      VAX/VMS   V3.2      Run-Time Library
      
      
      
      PROBLEM:
      
      The LIB$DAY Run-Time Library service "incorrectly"  assumes  the  year
      2000 is a leap year.
      
      
      RESPONSE:
      
      Thank you for your forward-looking SPR.
      
      Various system services, such as SYS$ASCTIM assume that the year  2000
      will  be  a  leap  year.   Although one can never be sure of what will
      happen at some future time, there is strong historical  precedent  for
      presuming  that the present Gregorian calendar will still be in affect
      by the year 2000.  Since we also hope that VMS will still be around by
      then, we have chosen to adhere to these precedents.
      
      The purpose of a calendar is to reckon time in advance,  to  show  how
      many  days  have  to  elapse  until a certain event takes place in the
      future, such as the harvest or the release of VMS  V4.   The  earliest
      vcalendars,  naturally,  were  crude  and  tended  to be based upon the
      seasons or the lunar cycle.
      
      The calendar of the Assyrians, for example, was based upon the  phases
      of  the  moon.  They knew that a lunation (the time from one full moon
      to the next) was 29 1/2 days long, so their lunar year had a  duration
      of  364  days.   This  fell  short of the solar year by about 11 days.
      (The exact time for the solar year is approximately 365 days, 5 hours,
      48  minutes,  and  46  seconds.)  After 3 years, such a lunar calendar
      would be off by a whole month, so the Assyrians added an  extra  month
      >from  time  to time to keep their calendar in synchronization with the
      seasons.
      
      The best approximation that was possible in antiquity  was  a  19-year
      period, with 7 of these 19 years having 13 months (leap months).  This
      scheme was adopted as the basis for the religious calendar used by the
      Jews.   (The  Arabs  also  used  this  calendar until Mohammed forbade
      shifting from 12 months to 13 months.)
      
      When Rome emerged as a world  power,  the  difficulties  of  making  a
      calendar  were  well  known,  but  the  Romans complicated their lives
      because of their superstition that even numbers were  unlucky.   Hence
      their  months were 29 or 31 days long, with the exception of February,
      which had 28 days.  Every second year, the Roman calendar included  an
      extra  month  called  Mercedonius of 22 or 23 days to keep up with the
      solar year.
      
      Even this algorithm was very poor, so that in 45 BC,  Caesar,  advised
      by  the  astronomer Sosigenes, ordered a sweeping reform.  By imperial
      decree, one year was made 445 days long to bring the calendar back  in
      step  with  the  seasons.  The new calendar, similar to the one we now
      use was called the Julian calendar (named after Julius Caesar).   It's
      months  were  30 or 31 days in length and every fourth year was made a
      leap year (having 366 days).  Caesar also decreed that the year  would
      start with the first of January, not the vernal equinox in late March.
      
      Caesar's year was 11 1/2 minutes short of the calculations recommended
      by  Sosigenes  and  eventually the date of the vernal equinox began to
      drift.  Roger Bacon became alarmed and sent a note to Pope Clement IV,
      who  apparently  was  not  impressed.   Pope  Sixtus  IV  later became
      convinced that  another  reform  was  needed  and  called  the  German
      astronomer,  Regiomontanus,  to  Rome  to  advise him.  Unfortunately,
      Regiomontanus died of the plague shortly thereafter and the plans died
      as well.
      
      In 1545, the Council of Trent authorized Pope Gregory XIII  to  reform
      the  calendar  once  more.   Most of the mathematical work was done by
      Father Christopher Clavius, S.J.  The immediate  correction  that  was
      adopted  was  that Thursday, October 4, 1582 was to be the last day of
      the Julian calendar.  The next  day  was  Friday,  with  the  date  of
      October  15.   For  long  range  accuracy,  a formula suggested by the
      Vatican librarian Aloysius Giglio was adopted.   It  said  that  every
      fourth  year  is  a  leap  year  except for century years that are not
      divisible by 400.  Thus 1700, 1800 and 1900 would not be  leap  years,
      but  2000  would  be a leap year since 2000 is divisible by 400.  This
      rule eliminates 3 leap years every 4 centuries,  making  the  calendar
      sufficiently  correct  for  most  ordinary purposes.  This calendar is
      known as the Gregorian calendar and is the one that we now use  today.
      (It  is  interesting  to note that in 1582, all the Protestant princes
      ignored the papal decree and so many countries continued  to  use  the
      Julian  calendar  until either 1698 or 1752.  In Russia, it needed the
      revolution to introduce the Gregorian calendar in 1918.)
      
      This explains why VMS chooses to treat the year 2000 as a leap year.
      
      Despite the great accuracy of the Gregorian calendar, it  still  falls
      behind very slightly every few years.  If you are very concerned about
      this problem, we suggest that you tune in  short  wave  radio  station
      WWV,  which  broadcasts  official  time  signals for use in the United
      States.  About once every 3 years, they declare a leap second at which
      time  you  should be careful to adjust your system clock.  If you have
      trouble picking up their signals, we suggest you  purchase  an  atomic
      clock (not manufactured by Digital and not a VAX option at this time).
      
                               END OF SPR RESPONSE