Nejc Puš
IWC Schaffhausen

The Calendar of a Watch

Nejc Puš
IWC Schaffhausen
We often notice a little window on wristwatches, where numbers from 1 to 31 interchange every 24 hours; on some of them, the days of the week, the month and the year are added. However, not all months are equally long and not every year has 365 days. Because of the six-hour difference between the calendar and the solar years, every fourth calendar year has 366 days, and in every 400 years, three leap years are not counted in our measurement of time.

Considering all these calendar changes, one wonders how are the watchmaking masters able to “program” the mechanical timepieces centuries in advance? 

Calendar Complications in this Article

The Moonphase Complication


Everything that offers an additional piece of information on tracking time besides hours, minutes and seconds, is called a complication. The simplest production of the calendar is to show the moon phases, because unlike calendar’s months and years, every moon cycle lasts exactly 29.53 days. For the depiction of the moon, as seen lit in the night sky, a disk with two bright circles is placed under specifically designed window on the dial. The disk depicts the moon and in one lunar cycle, it makes half a turn.


The calendar’s date is displayed in the date window on the dial, under which a disk with 31 numbers rotates. There is also a version with a big date display, where two disks compose the date’s number; the left disk with numbers from 1 to 3, and the right with the numbers from 0 to 9. Another known version has the date displayed on a subsidiary dial, where you can read the date with an additional hand that is moving on the circumference of the dial. What all versions of the ordinary calendar have in common is that they must be set again at the end of each month that does not have 31 days. 

Calendar with digital indicators of date and month
The simplest production of the calendar is to show the moon phases, because unlike calendar’s months and years, every moon cycle lasts exactly 29.53 days. 

The Annual Calendar


In order to avoid resetting the time piece every other month, the watchmaking masters had upgraded the complication back in the 19th century, so that the disk with the date skips the number 31 at the end of the 30-day months. But this medium complication has a drawback: it counts 31 days in February and it does not distinguish between leap years and common years.


The annual calendar, therefore, has to be reset once a year – at the end of February. Sophisticated yearly calendars have a 1461-day setting, which means they have to be reset (adjusted) once every four years.   

The Perpetual Calendar


The most sophisticated variation of the wristwatch’s calendar is called a perpetual calendar. These highly complex time pieces correctly display the date, day, month and year and, unlike the annual calendar, take into account the fewer number of days in February and even the leap years. Despite the complexity of the mechanism, the complication of perpetual time is not programmed for the three occasions when every 400 years, the leap years are omitted.

If you are a proud owner of a perpetual calendar, your grandchildren will have to reset the wristwatch again in 2100.
IWC Portugieser Perpetual Calendar

The Equation of Time

All the effort of the watchmaking masters to produce a mechanism, which by itself would follow the continuous changes in the number of days of a month or a year, is the consequence of the difference in length between the calendar and solar year. An upgraded complication of the perpetual calendar not only shows the ‘ordinary’ time on a dial, but it also indicates the difference between the solar time and the calendar time.


IWC Portugieser SIderale