Bug report... - Y2K Classics
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From: Peter Langston <psl>
Date: Wed, 7 Apr 99 11:52:08 -0700
Subject: Bug report... - Y2K Classics
Forwarded-by: David H Elrod <email@example.com>
[The following was taken from an actual bug report that I have seen myself.
It shows an edited bug report from a customer and an edited response. I did
the editing, to protect the customer and company involved. - dhelrod]
CUSTOMER BUG REPORT:
The [...]Library service "incorrectly" assumes the year 2000 is a leap
Thank you for your forward-looking bug report.
Various system services, such as [the one you reported] 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 [our system] 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 [the next version of our system]. 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). 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 [our system] chooses to treat the year 2000 as a leap
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 [our
company] and not a [system] option at this time).
© 1999 Peter Langston