Timepieces with indications of date with or without day and month are generally known as Calendar Watches.
Based on the type of horological complication incorporated, the calendar watches are available in Date Only, Day+Date, Date+Month and Day+Date+Month Versions. The watches that display day of the week, date and month details are again categorized as Triple/Full calendar, Annual Calendar and Perpetual Calendar. Some high complication watches come equipped with any of the calendar functions mentioned above.
Most of us often fail to differentiate between the various types of calendar watches. To understand the calendar watch complications better, we need to go through the Gregorian calendar, the most used civil calendar in the world.
Used all over the world, the Gregorian calendar is a solar calendar with 12 months. It has 365 days in a normal year and 366 days in a leap year.
In Gregorian calendar, numbers of days in a month vary from 28 to 31 days. January, March, May, July, August, October & December months have 31 days each; April, June, September & November months have 30 days each and the month of February has either 28 days in a normal year or 29 days in a leap year.
A Complete Calendar (Quantième Complet in French) watch displays date, day and months with or without moonphase. These types of timepieces are also known as Triple or Full Calendar watches. These watches need manual date adjustment in between the months with different number of days.
In complete/full/triple calendar watches, the date wheel complication reads all months as 31 days month. On 31’st midnight, the date automatically changes to 1’st of the next month. However in case of months with lesser than 31 days, the date will not change to 1’st day of next month in the midnight of last day of the month, so the watch needs correction.
For example, when the date changes from 30th June to 1’st July, the owner of the watch has to manually adjust the date with the help of crown or corrector in order to set the correct date. This type of watch needs manual adjustment of date 05 times in a year: 28/29 Feb to 01st March, 30 April to 01st May, 30 June to 01st July, 30th September to 01st October and 30th November to 01st December.
An Annual Calendar (Quantième Annuel in French) is a watch that shows the date, day of the week and month but requires just one adjustment per year in the transition from February to March.
In an Annual calendar watch, the date wheel complication is mechanically programmed for all months with 30 or 31 days so that from March to January, all month durations can be correctly indicated.
Usually, the annual calendar watch complication counts February as a 30 days month. So, in February, the date indication requires manual correction because according to the Gregorian calendar this particular month has only 28 or 29 days. To move date display from 28th February (29th in Leap year) to 1’st Mach, the owner of the watch needs to manually adjust the watch. As the name suggests, the Annual calendar watch requires manual date correction once in every year.
In some annual calendar watches, the date mechanism is mechanically programmed to count February as a 29 days month. In this condition, once a year, or rather, three times every four years, at the end of February, the wearer of the watch needs to set the date manually. This type of annual calendar watch doesn’t require any manual correction of date during leap years.
Another variant in this category, the calendar watch that displays day, date and month, but needs manual date adjustment once in every leap year known as Quadriannual Calendar. Here, the date mechanism is mechanically programmed to count February as a 28 days month. In this case, date automatically changes from 28th February to 1’st March every year, but the wearer has to manually set the date for 29th February in every leap year.
The watch complication that displays date, day, month and leap years is generally known as Perpetual Calendar. But technically, the answer is correct only if the date doesn’t need any manual adjustment for life time, more exactly till the next centurial year which is not a leap year. As of now, the next centurial year in Gregorian calendar is AD 2100.
To understand the Perpetual Calendar (Quantième Perpétuel in French) complication, we should know the significance of Leap years. A regular Gregorian year consists of 365 days, but in certain years known as leap years, a leap day is added to February. Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100, but these centurial years are leap years if they are exactly divisible by 400. For example, the years 1700, 1800, and 1900 are not leap years, but the year 2000 is.
The date complication in a perpetual calendar watch is designed to understand the differences in days of all 12 months in normal years and leap years in which the month of February has 29 days. So the calendar complication doesn’t need any manual correction of date even in leap years. This makes the date function “perpetual” in nature so this type of complication watch is known as a perpetual calendar.
Most contemporary perpetual calendar watches however need a correction in February 2100 because as per the Gregorian calendar, AD 2100 is not a leap year, and the February month has only 28 days. The perpetual calendar mechanism however reads the year 2100 as a leap year so the owner of the watch has to manually adjust the date from 28th February to 1’st March to synchronize with the traditional Gregorian calendar.
The Business Calendar (Semainier) is a calendar watch that also displays week of the year (1 to 52 weeks).