Perpetual calendar (watch)

^{(In French: calendrier perpétuel (montre à))}

A watch whose perpetual calendar (see complication) automatically takes the number of days in the month into account: 30 or 31 and the 28 or 29 days of February for ordinary and leap years. Unless it takes into account century years that are not leap years, it will need adjusting in 2100, 2200 and 2300 but not in 2400. A 48-month dial, derived from pocket watches, corresponds to three ordinary years and one leap year. On the more legible 12-month dial, ordinary and leap years are shown by a hand or aperture.

Some perpetual calendars can include the following additional functions:

• Week number this being virtually the same as the interval between two consecutive phases of the moon (see complication).
• Year corresponding to the order of years in a religious era, whether Christian or another faith (see complication).
• Sunrise and sunset for a given location (see complication) when a perpetual calendar mechanism drives the sunrise and sunset wheels, these indications are said to be perpetual.
• Sidereal hour equal to one-twenty-fourth of the sidereal day (see complication), this being the interval between two successive transits of a star over the meridian.

Exceptional watches can give other astronomical indications such as the declination of the Sun (angular distance north or south from the celestial equator), the apparent movement of the planets, the line of node to forecast eclipses, a star chart for a given location, etc.