This very useful complication takes
account of the number of days in the month.
It is capable of recognising a 30-day month
without manual intervention.
Our Gregorian calendar with its months of variable length creates a challenge for watchmakers. For a long time, watches with date indication had to be corrected five times a year by hand, at the end of months lasting less than 31 days.
The annual calendar watch indicates the date of the month, automatically taking account of the variation in number of days between the months. Unlike the perpetual calendar, the annual calendar does not take account of the month of February – treating it as a 30-day month – nor of leap years. It therefore has to be manually adjusted once a year, at the end of February.
Every day towards midnight the hour hand triggers the date indicator via a gear train. The differences in length between the months are taken into account using a cam wheel possessing lobes and notches of varying depths. This secondary date mechanism causes an additional jump when the month has 30 days.
Certain timepieces display the date on the main dial using an extra hand, while others have a dedicated aperture.