The ABC of Calendar Watches: Which types are there?

The calendar is without a doubt one of the most popular complications out there, not least thanks to its great usefulness. Besides the time of day, the date is the information that one needs to have at hand most often in everyday life. Nonetheless, there is a huge range of calendars, ranging from fairly basic to technically most impressive.

We have compiled the five most common calendar mechanisms (in increasing complexity) and briefly point out their main characteristics. Let’s dive right in.

Simple Calendar

The simple calendar is – no surprise here – fairly simple and therefore found in mechanical watches the most often. Usually the date window sits at 3 o’clock, under which a circular date disk with numbers from 1 to 31 rotates. Since the simple calendar does not differentiate between months with 30 or 31 (let alone 28) days, it must be adjusted after every February, April, June, September and November.

Tl;dr: The simple calendar only shows the date of the month and has to be corrected five times a year.

Full Calendar

The full calendar (also called complete calendar or triple calendar) requires the same number of manual corrections as the simple calendar. The difference, however, lies in a more sophisticated display: In addition to the day of the month, a full calendar also shows the month and the day of the week. (A moon phase is commonly used in combination with a full calendar as well.)

Tl;dr: A full calendar extends the simple calendar by the month and weekday display (and sometimes a moon phase).

Annual Calendar

The annual calendar is the first of the calendars in this list to have a “mechanical memory” so to speak, taking into account the difference between months with 30 and 31 days (except for February). Therefore, a correction must only be made every year at the end of February.

Tl;dr: A watch with an annual calendar only needs to be corrected once a year, namely at the end of February.

Quadrennial Calendar

The quadrennial calendar (also called semi-perpetual or four-year calendar) is an improvement to the annual calendar and a compromise to the calendar that follows subsequently. When worn regularly, a quadrennial calendar runs correctly for four years without any intervention by the wearer. Only when a leap year with 366 days occurs, i.e. when there is a 29th of February, the calendars requires a manual readjustment. To put it differently: The shorter duration of February, which the annual calendar ignores, is taken into account with quadrennial calendars.

Tl;dr: Quadrennial calendars only need to be corrected manually every four years, namely when a leap year occurs.

Perpetual Calendar

Saving the best for last, perpetual calendars represent the top end of the flagpole: A perpetual calendar is smart enough to know when a month is just 30 days long or when a leap year occurs. Even if full calendars and perpetual calendars may seem to look alike at first glance, the latter ones are one of the five grande complications due to their insane complexity. It must be noted, however, that nearly all mechanical perpetuals on the market still need a nudge on the first year of three out of every four centuries, as the underlying Gregorian calendar system is still not 100% precise (although admittedly pretty darn close).

Tl;dr: The perpetual calendar, the supreme discipline among wristwatch calendars, takes all months of different lengths as well as leap years into account. It does not have to be corrected manually until the year 2100.