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Year 2000

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Content starts here

To Leap or Not to Leap?

This was never a question for OpenVMS!

For anyone who still wonders whether the year 2000 is a leap year, we have dusted off the archives from 13-OCT-1983 to retrieve this reply from DIGITAL's Stanley Rabinowitz to a Software Performance Report (SPR). The report, filed against VAX/VMS Version 3.2, claimed the LIB$DAY Run-Time Library service assumed incorrectly that the year 2000 was a leap year.

Stan's now famous response demonstrates that OpenVMS has been forward thinking in terms of the year 2000 long before the current hoopla began!

NOTE: The astute reader may notice a technical typo in the SPR response. In the interests of preserving this archival document -- and permitting future readers the thrill of the hunt! -- we have intentionally left this error unedited.


			    D I G I T A L

                           SPR ANSWER FORM

SPR NO. 11-60903

SOFTWARE:  VAX/VMS  V3.2      VAX/VMS   V3.2      Run-Time Library


The LIB$DAY Run-Time Library service "incorrectly"  assumes  the  year
2000 is a leap year.


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
calendars,  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

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

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