Calendar reform, properly calendrical reform, is any significant revision of a calendar system. The term sometimes is used instead for a proposal to switch to a different calendar. Most calendars have several rules which could be altered by reform: \n* Whether and how days are grouped into subdivisions such as months and weeks, and days outside those subdivisions, if any. \n* Which years are leap years and common years and how they differ. \n* Numbering of years, selection of the epoch, and the issue of year zero. \n* Start of the year (such as the December solstice, January 1, March 1, March equinox, March 25). \n* If a week is retained, the start, length, and names of its days. \n* Start of the day (midnight, sunrise, noon, or sunset). \n* If months are retained, number, lengths, and names of months, \n* Special days and periods (such as leap day or intercalary day). \n* Alignment with social cycles. \n* Alignment with astronomical cycles. \n* Alignment with biological cycles. \n* Literal notation of dates.

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