"Simple calendar" watches show the date without taking into account variations in the length of the months. Because of this, they have to be manually corrected five times a year. In addition to the date, many simple calendar watches indicate the day, month and possibly moon phases too.
Calendar indications are among a watch's most useful functions for every day. They were an early feature of portable timepieces, appearing even before the minute hand. Based on a differential gear mechanism from the hour wheel, the "simple" calendar does not take into account months with fewer than 31 days nor leap years. For this reason it must be manually corrected five times a year, at end February, April, June, September and November, using a button or the winding stem.
The calendar mechanism can be "trailing" (gradual date change), semi-instant (taking around an hour and a half) or instant (on the stroke of midnight). The majority of simple calendars only give the date, usually in a small aperture. Some though display additional information, such as the day, month or moon phases, either by means of hands or on discs that are visible through apertures.
See "Simple calendar watches" in the Glossary.