Calendar reform or 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 design. Principles The prime objective of a calendar is to unambiguously identify any day in past, present and future by a specific date in order to record or organize social, religious, commercial or administrative events. Recurring periods that contain multiple days, like weeks, months and even years, are secondary, convenient features of a calendar. Most cultures adopt a primary dating system, but different cultures have always needed to align multiple calendars with each other, either because they coexisted in the same space (e.g. secular and religious groups with different demands) or had established trading relations. Once specified, a calendar design cannot be altered without becoming a new design. If a proposed design is sufficiently close to the legacy one, i.e. compatible with it, a reform of the local calendar system is possible without disruption. Examples of this include the changes to the Chinese calendar due to problems between regions in China and practical changes in religious calendars such as the Islamic calendar where the start of the month is now derived from astronomical data rather than sightings by religious leaders. Some design changes, however, will yield date identifiers different from the previous design for some days, often in the distant past or future. The calendar system must clarify whether dates are changed to the new design retroactively (using a proleptic calendar) or whether the design in use then and there shall be respected. Calendar schisms happen if not all cultures that adopted a common calendar system before perform a calendar reform at the same time. If date identifiers are similar but different, confusion and mistakes are unavoidable. Most calendars have several rules which could be altered by reform: Whether and how days are grouped into subdivisions such as months and weeks, and days outside those subdivisions, if any. Which years are leap years and common years and how they differ. Numbering of years, selection of the epoch, and the issue of year zero. Start of the year (such as the December solstice, January 1, March 1, March equinox, Lady Day). If a week is retained, the start, length, and names of its days. Start of the day (midnight, sunrise, noon, or sunset). If months are retained, number, lengths, and names of months.