# 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.

DayOfWeek[{year,month,day},Calendar->cal] | |

give the day of the week on which the given date occurred using the calendar system cal | |

DaysBetween[{year_{1},month_{1},day_{1}},{year_{2},month_{2},day_{2}},Calendar->cal] | |

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

DaysPlus[{year,month,day},n,Calendar->cal] | |

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

CalendarChange[{year,month,day}, cal_{1},cal_{2}] | |

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 English‐speaking 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 you 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 30‐year cycle. The Islamic calendar began on the Hegira, which was July 16, 622 in the Julian calendar.

EasterSunday[year] | give the date of Easter Sunday in the Gregorian system |

EasterSundayGreekOrthodox[year] | give the date of Easter Sunday according to the Greek Orthodox Church using the Gregorian system |

JewishNewYear[year] | give the date of the Jewish New Year occurring in Gregorian years between 1900 and 2099 |