DEC Y2K Report

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

last update: 10 April 2000