I'm sick of hearing about the Mayans, so here's an oldie but goodie:
This information comes from the original (pre-Motif) DECwindows help
file which accompanied VMS version 5.3. It was written by Marios
Cleovoulou and is copyright © 1988, 1989 by Digital Equipment
As decreed by Pope Gregory XIII, October 4, 1582, was followed by
October 15, 1582. Thus ended the 1,600 year reign of the Julian calendar
upon which the Gregorian calendar is based, and thus began the calendar
which DECwindows Calendar uses to measure time.
Calendars based on sun and moon movement were used even by the ancients,
but the first reasonably accurate one was the 365 1/4- day cycle
calculated by the Greek Sosigenes. This was the calendar authorized by
Julius Caesar in 46 BC. The Julian calendar (not to be confused with the
Julian period; see below) had three years of 365 days each, followed by
a fourth year of 366 days.
The 365 1/4-day cycle was more accurately defined in 730 AD by the
Venerable Bede, an Anglo-Saxon monk, who shortened the time by
11 minutes, 14 seconds. This accumulates to a whole day's error every
128 years, or a little more than three days every 400 years. This being
the Dark Ages, nothing was done to adjust the Calendar, despite Roger
Bacon sending a note to Pope Clement IV, informing him of the drifting
of the date for the vernal equinox. Later, Pope Sixtus IV did become
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
Thursday, October 4, 1582 was the next time the calendar was adjusted.
This last day of the Julian calendar was followed by Friday, October 15.
So began the Gregorian calendar that we use today, named after Pope
Gregory XIII. He commissioned the mathematician Father Christopher
Clavius, S.J., to do the necessary calculations, having been authorized
to reform the calendar by the Council of Trent in 1545.
The Vatican librarian Aloysius Giglio provided a formula for long- range
accuracy. He suggested that every fourth year be 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, because 2000 is divisible by
400. This rule eliminated three leap years every four centuries, making
the calendar sufficiently correct for most ordinary purposes.
Political Acceptance in Europe
Italy, Portugal, and Luxembourg immediately adopted the new calendar. By
1584, Belgium, parts of the Netherlands, Switzerland, and most Catholic
German states had joined, and by 1587, so had Hungary. It was not until
1699-1700 that these countries were joined by the rest of the
Netherlands, Denmark, and the Protestant German countries.
By the time the British imposed the calendar on all its possessions, in
1752, 11 days needed to be lost. September 2, 1752, was thus decreed to
be followed by September 14. In addition, New Year's day was moved back
from March 25 to January 1. (For example, before, March 24, 1700 had
been followed by March 25, 1701). Among other repercussions, this moved
Washington's birth date from February 11, 1731, to February 22, 1732.
The following year, 1753, Sweden too adopted the calendar.
In 1793, the French Revolutionary government adopted a calendar of 12
onths of 30 days each, with five extra days in September (six on leap
years). The Gregorian calendar was reinstated in 1806 by Napoleon.
Political Acceptance World Wide
Adoption of the calendar in countries outside Europe and its Crown
possessions occurred much later, and often in conjunction with political
upheaval: Japan in 1873, Egypt in 1875, China in 1912, and Turkey in
In 1918, Russia's revolutionary government decreed that January 31,
1918, would be followed by February 14, 1918.
Religious Acceptance Worldwide
German Protestants used the old calendar until 1776, three quarters of a
century after their countries had adopted the Gregorian system.
Sweden retained the old Easter rules for 90 years after switching to the
Gregorian calendar, and many Middle Eastern Christian sects still retain
the Julian calendar.
The Russian Orthodox Church still follows the Julian system.
The Julian Period
Astronomers use the Julian period because it is convenient to express
long time intervals in days rather than months, weeks and years. It was
devised by Joseph Scaliger, in 1582, who named it after his father
Julius, thus creating the confusion between the Julian (Caesar) calendar
and the Julian (Scaliger) period.
Julian Day 1 began at 12:00 noon, January 1, 4713 BC. This date was
thought by some to correspond approximately to the beginning of the
universe. Certainly it predated any known astronomical events known in
the 16th century without resorting to negative times. Scaliger decided
on the actual date on the grounds that it was the most recent
coincidence of three major chronological cycles:
- The 28-year solar cycle, after which dates in the Julian calendar (for
example September 27) return to the same days of the week (for example
- The 19-year lunar cycle, after which phases of the moon return to the
same dates of the year.
- The 15-year indiction cycle, used in ancient Rome for tax regulation.
It takes 7980 years to complete the cycle. Noon of January 1, 1988,
marks the beginning of Julian Day 2447161.
The Julian period is also of interest because of its use as a time base
by the VMS operating system.
VMS and the Julian Period or:
Why VMS regards November 17, 1858,
the beginning of time...
The modified Julian date adopted by SAO (Smithsonian Astrophysical
Observatory) for satellite tracking is Julian Day 2400000, which turns
out to be November 17, 1858.
SAO started tracking satellites with an 8K (nonvirtual) 36-bit IBM 704
in 1957, when Sputnik went into orbit. The Julian day was 2435839 on
January 1, 1957. This is 11225377 octal, which was too big to fit into
an 18-bit field. With only 8K of memory, the 14 bits left over by
keeping the Julian date in its own 36-bit word would have been wasted.
They also needed the fraction of the current day (for which 18 bits gave
enough accuracy), so it was decided to keep the number of days in the
left 18 bits and the fraction of a day in the right 18 bits of one word.
Eighteen bits allows the truncated Julian day (the SAO day) to grow as
large as 262143, which from November 17, 1858, allowed for seven
centuries. Possibly, the date could only grow as large as 131071 (using
17 bits), but this still covers three centuries and leaves the
possibility of representing negative time. The 1858 date preceded the
oldest star catalogue in use at SAO, which also avoided having to use
negative time in any of the satellite tracking calculations.
Ultrix (Unix) Time Origins
The beginning of time for Ultrix systems is:
Thursday January 1 00:00:00 1970
The reason for this date being chosen is that this was the year that
UNIX, the "father" of Ultrix, was firstreleased.
Thus dates prior to 1970 are BU; 1970 and later dates are AU.
History and DECwindows Calendar
If you read the topics concerning the political and religious acceptance
of the Gregorian calendar, you will see that there is a problem: there
are many dates for the conversion from the Julian to the Gregorian
calendar. For example, the British (and therefore the Americans)
converted in September 1752, so British and American Calendar users
might expect Calendar to show September 2, 1752, to be followed by
September 14, 1752. However, a Russian user would expect to see this
jump between January 31, 1918, and February 14, 1918.
DECwindows Calendar conforms to the date of the original decree,
therefore no days have been lost since Friday, October 15, 1582, nor
since the beginning of DECwindows Calendar time: January 1, 1600. Thus
for everyone except for users from the majority of the Catholic European
countries, which all converted before this date, there will appear to be
an "error" in Calendar, where the conversion actually took place. This
generalization was felt to be acceptable for an application not
specifically designed for historians.
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