IP: Leap Year 2000 Explained

[Dave Farber <farber@cis.upenn.edu>]

>Date: Tue, 29 Feb 2000 13:34:32 -0800
>From: Gary Delp <gdelp@siara.com>
>To: Dave Farber <farber@cis.upenn.edu>
>Subject: [Fwd: [Fwd: Leap Year 2000 Explained]]
>
>
>-------- Original Message --------
>Subject: death_to_vms
>Date: Wed, 24 Mar 1999 08:28:12 -0800
>
>The year 2000 is a leap year.
>
>Some years ago, a Digital employee called Stan Rabinowitz was called
>upon to answer a customer's complaint that VMS "incorrectly" handled
>the year 2000 as a leap year.  His extremely thorough response has
>since become quite famous in VMS circles, as it is probably the only
>SPR response to mention Sosigenes, Regiomontanus and the Council of
>Trent.  The following is the original draft of this response, although
>how it left Digital is a matter of some conjecture.  Digital
>management made him remove the references to radio station WWV, atomic
>clocks and VMS V4 before sending it to the customer.
>
>It has been reformatted slightly for easy viewing.
>
>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 effect
>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 354 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).  Its
>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