A calendar 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:

Historically, most calendar reforms have been made in order to synchronize the calendar in use with the astronomical year (either solar or sidereal) and/or the synodic month in lunar or lunisolar calendars. Most reforms for calendars have been to make them more accurate. This has happened to various lunar and lunisolar calendars, and also the Julian calendar when it was modified into the Gregorian calendar.

The fundamental problem of the calendar is the imperfect divisibility of whole numbers into an irrational number (fitting whole days into a month; fitting whole days or whole months into a year). The physics of orbital mechanics does not phase-lock the rotation of the Earth (the day) to its revolution (the year), nor the rotation of the Earth (the day) to the revolution of the Moon (the month). Therefore any attempt to divide a month into days or a year into days will leave a fractional remainder of a partial day. Likewise, any attempt to divide a year into months will leave a fractional remainder of a partial month. Such remainders accumulate from one period to the next thereby driving the cycles out of synch.