What is a Perpetual Calendar watch and how does it work?

What is a Perpetual Calendar watch and how does it work?

The early "calendar" watches, dating from the 16th century, were equipped with a mechanism giving the day, date and month in addition to the hour which was still imprecise at that point in time.

In a calendar watch, the days and months follow sequentially but the same cannot be said of the dates which are either 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, i.e. the ist 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 as opposed to "simple calendar" timepieces. 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. Regarding leap years, February 29 has been deleted in the years 1700, 1800 and 1900. It won't be deleted in 2000 but will be in 2100, thus today's ads for perpetual calendar watches are right in their claims that these models will not have to be corrected for over a century. The actual duration of a year is 365.2422 days. The perpetual calendar counts the year as having 365.25 days while the simple calendar counts 12 x 31 = 372 days making it necessary to remove 6 or 7 days every year.


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