The author of this was "Stanley
Rabinowitz", famous in the DEC world
as the author of the SMG$ library, the
WHAT utility and many other parts of
VMS. The copyright of this is
therefore probably his, but he has now
left DEC.
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 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 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