Calendar reform

Calendar reform is the redesign or adjustment of a calendar to correct its drift from astronomical cycles, improve its structure, or meet social and administrative needs.

Gregorian calendar: The solar calendar introduced by Pope Gregory XIII in 1582 to correct drift in the Julian calendar. Its adoption is the best-known correction of accumulated calendar drift.
Tropical year: The average interval between successive returns of the Sun to the same seasonal position, about 365.2422 days. Its length sets the target for a calendar intended to keep seasons aligned.
Roman calendar: The changing system of timekeeping used in ancient Rome, including its early lunar structure and later Julian form. Roman calendar irregularities created the need for Caesar’s major restructuring.
Gregorian calendar adoption: The gradual acceptance of the Gregorian calendar by states and communities after its introduction in 1582. Uneven adoption made the reform a prolonged political and religious transition.
Hijri calendar: A lunar calendar of twelve months used for Islamic religious dates, with years shorter than solar years. Its months follow lunar phases without remaining fixed to the seasons.
Julian calendar: The solar calendar established under Julius Caesar, using a leap day every four years. Its small annual excess produced the drift that prompted the Gregorian reform.
Leap year: A year with an extra day or other adjustment that keeps a calendar synchronized with an astronomical cycle. Leap-year rules regulate the mismatch between whole calendar days and the tropical year.
Julius Caesar: Roman political and military leader who introduced the Julian calendar in 46 BCE. His reform replaced a heavily disrupted Roman system with a solar calendar.
Calendar controversy: Public or institutional disputes over calendar systems, dates, or proposed changes to timekeeping. Reforms can provoke resistance when they alter familiar dates or religious observances.
Hebrew calendar: A lunisolar calendar that inserts leap months to keep Jewish months aligned with the seasons. Its intercalary months balance lunar dating and seasonal continuity differently from a solar calendar.

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