Gregorian and Julian Calendars
The  time taken for the Earth to orbit the sun is approximately 365.242198 days.
This  means that in order to avoid drifting from the solar year, a calendar must
correct for the extra 0.242198 day (05:48:45.9072) each year.

The  Romans  attempted  to accommodate this extra time with the Julian calendar,
established  by  Julius Caesar in 45 BCE. The Julian calendar added an extra day
to each fourth (leap) year, producing an approximation of 365.25 days.

By the sixteenth century, it became apparent that the Julian calendar was out of
synch with the solar year by well over a week.

The  Gregorian  calendar,  proposed by Neapolitan physician Aloysius Lilius, was
introduced  to  correct  for this error. In accordance with recommendations from
the  Council  of Trent (1545-1563), it was decreed by Pope Gregory XIII in papal
bull "Inter Gravissimas" on Feb 24, 1582.

In most of continental Europe, the new calendar came into effect on October 4th,
1582,  with  the  removal of the following 10 days. This meant that, in Rome for
example, the day following October 4th was October 15th. France followed shortly
after, on December 9th of the same year.

In  Great  Britain  (UK)  however,  the  new  calendar was not adopted until the
→British_Calendar_Act_1751←, which removed 11 days September 3-13 in 1752. Diff-
erent  states  in  the US adopted the changes at different times, depending upon
their allegiance to Rome, Paris or London. See →gcr←.

All of this means that when converting historical dates in the Julian, as oppos-
ed  to  the  modern  Gregorian  calendar, we must be careful to specify when the
change took place. For example, specific dates in the seventeenth century do not
refer to the same day when quoted in French or British sources. The problem even
occurs in adjacent states in the US! See →gcr←.

Both  →days←  and  →date←  take the date of the Julian to Gregorian change as an
optional left argument. →gcr← lists arguments for various locations.

The Gregorian calendar approximates the solar year as 365+97÷400 = 365.2425 days
which means that it takes approximately 3,300 years to shift one day relative to
the true solar year.

The approximation 356+97÷400 is achieved by an injecting an extra 97 "leap" days
every 400 years, using the rule:
                                          Extra days / 400 years
    Each year in 400 has 365 days,  ·   ·   ·   ·   ·   ·      0
    ·   except that every 4 years is a leap year,   ·   ·   +100
    ·   ·   except that every 100 years isn't,  ·   ·   ·     -4
    ·   ·   ·   except that every 400 years is. ·   ·   ·     +1
The rule could be extended to give ever-closer approximations to the true (corr-
ect  to  9  sig.  figs)  solar year of 365.242198 days. Astronomer John Herschel
(1792-1871), and others, proposed a closer approximation of:

     365 + 969 ÷ 4000 → 365.24225  days.

This  correction would require that every 4,000 year period include only 969 ex-
tra  days, compared with the Gregorian calendar's 970, suggesting the additional
    ·   ·   ·   ·   except that every 4,000 years isn't.

To date, no-one has implemented Herschel's suggestion.

See also: mayan date days gcr
See also: British_Calendar_Act_1751

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