DEC_Calendar_SPN

Posted-date: 14-OCT-1983 @ 03:08:00
Subject: My SPR answer (for your review)

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

                            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 V6. 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 th e 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