Perpetual calendar: The watch with a marvellous brain

May 13, 2013
Perpetual calendar: The watch with a marvellous brain
Perpetual calendar: The watch with a marvellous brain

The conventional wristwatch with a date function performs 31 24-hour cycles before starting all over again. The concept of building a watch that could “know” when it was a month with only 30 days or a leap year was one of the classic challenges facing the world of horology, till the perpetual calendar wristwatch was born. Its fully automatic calendar adjusts the end of each month, including the months of a leap year.

In watchmaking, "perpetual calendar" describes a calendar mechanism that correctly displays the date on the watch 'perpetually', taking into account the different lengths of the months as well as leap days. The internal mechanism will move the dial to the next day. 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.

In a "simple" calendar watch, the days and months follow sequentially but the same cannot be said of the dates which are 28, 29, 30 or 31 depending on the month and whether it is leap year or not. In a "simple" calendar watch it is necessary to correct the date five times during the year­-on the first day of March, May, July, October and December.

Abrahain-Louis Breguet is usually credited with having invented the mechanism which made these corrections automatically. His invention led to today's "perpetual calendar" watches. These models are based on the Julian calendar rather than the Gregorian calendar in use today. As a result, leap years are not deleted at the end of three out of four centuries, thus making it necessary to correct the watch three times in 400 years.

A perpetual calendar gets around that by “knowing” how many days there are in a month. There are two types of perpetual – the semi perpetual which can handle everything except leap years, and the full perpetual which can also handle the 29th of February.

Most perpetual calendars also include a day, month and year feature in addition to the date – they have to know which month and year it is, so they may as well show it, and the day of the week doesn’t make things much more complicated. It’s also important to understand that while the vast majority of watches use the same mechanism to tell the time, there are a number of different ways to build perpetual movements.

Most perpetuals have separate mechanisms for date, day, month, etc. However, all of these are obviously linked together for regular operation – it is the ‘power’ that comes independently from the movement.

The secret to a perpetual is the cam that drives the months. This cam has three different types of markings on it – generally 7 ridges (for months with 31 days), 4 gaps / dents (for months with 30 days), and a larger dent with an additional rotating piece for February.

A lever pushes against this cam and then connects to the date wheel via a ‘finger’ or pawl that can engage with the teeth on the date wheel. The position of the lever (dictated by the ridge or gap on the month cam) will determine when the finger engages with the date wheel and hence when it forces the wheel to move to the 1st. Normally the date is changed by another finger that engages at midnight (or thereabouts) and moves the date forward one day. When the finger from the lever attached to the month cam engages it does so at the 31st and overrides the finger that usually moved the date, thereby moving the date to the 1st.

For February, the additional rotating piece that sits under the date wheel is essentially a way to adjust the depth of the dent. It has four different settings and rotates once per year (off of the month mechanism). For three years out of four the setting is deep enough for the finger to engage on the 28th, on the fourth it makes the dent slightly shallower and triggers the finger to engage on the 29th.

Many perpetual watches also include a season indication, and that also works off of the month wheel, although it generally rotates each month to indicate not just the season, but where in the season we are (1st month, 2nd month, 3rd month).