The DEC customer support representative replied with this incredibly interesting history of the calendar as we know it:

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 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 364 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).

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 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 364 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).