Accurate to the day The annual calendar and the perpetual calendar

On 24 February 1582, Pope Gregory XIII decreed the introduction of the Gregorian calendar that was named after him. The previously valid Julian calendar was based on a year duration of 365.25 days. Compared to the solar year, the Julian year was 11 minutes and 14 seconds too long, which, in the course of several centuries, produced a growing deviation from the actual progression of the sun. In the 16th century, the difference had already grown to ten days.

Today, the Gregorian calendar is the most commonly used calendar in the world. It attempts to map the precise astronomical duration of a year (a solar year) of 365.24219 days and has the following rules:

Years divisible by four are extended with a leap day, giving the year an average of 365.25 days.
The leap day is omitted in years that are divisible by 100. The year has an average of 365.24 days.
Finally, the leap day is again added to years divisible by 400. Now, a year has an average duration of 365.2425 days, coming very close to the solar year.

Around the year
Save the date: 1 March 2100

Around the year

An annual calendar unites a number of calendar functions. Apart from indicating the time, it has an outsize date and displays the day of the week, the month and the moon phases. An elaborate mechanism automatically recognises which months have 30 and 31 days. Once a year, at the transition from February to March, the display must be updated because the mechanism handles February as a month with 30 days.

Save the date: 1 March 2100

If the calendar mechanism of a mechanical watch is perpetual, it automatically displays the correct date, day of the week, month and moon phases. The watch not only emulates the durations of each month in the course of a year, but also knows that every fourth year is a leap year. An adjustment is only necessary after every full century when the Gregorian calendar omits the leap year. So according to this rule, the date then needs to be adjusted by one day on 1 March 2100. The same applies to the years 2200 and 2300. Since the leap day is added again in 2400 (see rule 3), your perpetual calendar will then run correctly again for 200 years. The next adjustment can wait until 1 March 2500.

The core element of the calendar mechanism is the 48-step cam which rotates only once every four years. It features recesses of various depths that are sampled by a finger. The deeper the recess, the sooner the mechanism switches to the first day of the next month. If the finger is not in a recess but instead glides along the outer circumference of the 48-step cam, the respective month has 31 days. The shallower recesses correspond to months with a duration of 30 days, and the deepest ones are reserved for the month of February when it has 28 days. A single mid-depth recess marks 29 February when it occurs in leap years.

The 48-step cam of the perpetual calendar is sampled by a finger. It is programmed with all month durations across a four-year period.

View of the perpetual calendar mechanism with moon-phase display

The durations of the months across the four-year cycle, including leap-year adjustment (29 February), are sampled at the 48-step programme disc and transferred to the complex calendar mechanism.

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