- Simple calendar: shows the date, i.e. the number, in order, of each day in the month. Certain simple calendars also display the names of the months. A simple calendar mechanism uses a single disc display, or two discs when it incorporates a Large Date function.
- Full calendar: shows the date, the day of the week, the month and moon phases (new and full moons separated by the first and last quarters). A complete calendar mechanism uses three discs, four discs when it incorporates a Large Date function, and four when moonphases are added.
Whether full or partial, a simple calendar does not automatically take months with less than 31 days into account, or leap years, and must therefore be adjusted five times a year.
Information is displayed in letters and digits, or just digits, through one or more apertures, or by means of hands.
See complication, simple calendar watches.