Calendars are perhaps the most common variety of complication. From very basic day-date calibers to complex perpetual calendars, almost every major Swiss watch brand does some type of calendar execution. Here we will take a quick look at the most common calendar examples.
There are several types of calendar movements, with the most basic being a date window closely followed by the day-date. This movement is simply that, displaying only the day and date. The date of this complication is not tied to any month, therefore each full date cycle is 31 days. Corrections to the date must be made manually at the end of each month if the month ends in 28 or 30 days.
A complete calendar (sometimes called a triple calendar) displays the month, day and date. But much like the day-date complication, adjustments to the date must be made at the end of each month.
An annual calendar is an accurate yearly calendar that’s technically correct for all non-leap years. Annual calendars must be adjusted each year at the end of February as they do not account for leap year. The first annual calendar was patented by Patek Philippe in 1996.
A perpetual calendar is designed to be an accurate representation of the Gregorian calendar. What differentiates a perpetual calendar and an annual calendar is the leap year correction. A perpetual calendar takes into account an extra day in the yearly calendar every four years. While this might seem like a small distinction, it’s actually a tremendous difference in terms of watchmaking. Interestingly, perpetual calendar watches far out-date annual calendar watches by many, many decades.
Equation of Time
An equation of time is a really interesting calendar function that is generally associated with a perpetual calendar. An equation of time measures the difference between mean solar time, as measured by the watch itself and apparent solar time which “is based on the rotation of the earth and the position of the sun in the sky with respect to the local meridian.” The equation of time is a rare complication and its creation is reserved for the most talented manufactures in the world.