Calendar Package
This package provides a unified treatment of the basic calendar operations. The main idea is to treat the calendar as a generalized number system, so that days, weeks, months, and years are thought of as generalizing the digits of a number in a given base.
DayOfWeek[{year,month,day}]  give the day of the week on which the given date occurred 
DaysBetween[{ year_{1}, month_{1}, day_{1}},{ year_{2}, month_{2}, day_{2}}] 
 give the number of days between the given dates 
DaysPlus[{year,month,day},n]  give the date n days after the given date 
Calendar computations using the standard calendar.
The basic calendar utility is finding the day of the week.
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This computes the number of days between the given dates. The year 1900 was not a leap year.
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January 2, 1901 is 366 days after January 1, 1900.
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 give the day of the week on which the given date occurred using the calendar system cal 

 give the number of days between the given dates using the calendar system cal 

 give the date n days after the given date using the calendar system cal 

 convert the date given in cal_{1} into a date given in cal_{2} 
Calendar computations using specified systems.
The
Gregorian calendar has been in use in the Western world since 1582 by Roman Catholic countries, and since 1752 by Englishspeaking countries. The
Gregorian calendar counts leap years every year divisible by 4, except for centuries not divisible by
400, which are not leap years.
The calendar used before the
Gregorian calendar was the
Julian calendar. This system counts
every year divisible by 4 as a leap year. At present there is a disparity of 13 days between the calendars. The changeover between these two calendars was done by omitting a number of days, therefore one must account for this in computations.
The default calendar used in this package is the one used by England and her former colonies (e.g., the United States). It uses the
Gregorian calendar for dates starting with September 14, 1752, and the
Julian calendar for dates up to September 2, 1752. If you do not specify a system, this is the calendar used by
DayOfWeek,
DaysBetween, and
DaysPlus.
The
Julian calendar is valid to March 1, year 4, but not before then since the year 4 was
not a leap year.
The
Islamic calendar is used mainly to keep track of Islamic holy days. It is a purely lunar calendar and a year has either 354 or 355 days. The months do not correspond to the solar year and migrate over the solar year following a 30year cycle. The
Islamic calendar began on the Hejira, which was July 16, 622, in the
Julian calendar.
In the Julian calendar, the year beginning a century not divisible by 400 is a leap year.
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This gives the first day in the Islamic system, the date of the Hejira. The date is given in the Julian calendar because the Gregorian calendar was not used before 1582.
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Dates of holidays.
In 1945, Easter Sunday was on April 1.
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