The annual calendar automatically recognizes months of 30 and 31 days, but does not account for February: it requires only one manual correction per year, between late February and early March. It sits in the middle of the hierarchy of mechanical calendars: more sophisticated than the complete calendar, which requires correction every month under 31 days, yet less autonomous than the perpetual calendar, which handles leap years on its own for over a century.
The first annual calendar wristwatch is attributed to Patek Philippe, which introduced it in 1996 with the ref. 5035. The mechanical principle relies on a wheel programmed with different profiles for each month: teeth or cams physically distinguish long months from short ones, skipping the date correctly. February remains the blind spot of the system: the mechanism cannot distinguish on its own between a 28-day February and a 29-day one, because the leap year cycle requires four-year logic that the annual calendar does not incorporate.
Rolex developed its own version with the Saros system, patented and integrated into the 9001 caliber of the Sky-Dweller since 2012. The name echoes the astronomical cycle that governs eclipses. It works with a date-reading mechanism coordinated with a monthly disc, and requires correction on March 1st. There are variants with instantaneous, bidirectional correction, allowing you to adjust the date by advancing or rewinding without risking damage to the mechanism.
Between the annual and perpetual calendar, there is also the quadrennial calendar, less common: it automatically corrects for four years, requiring intervention only at the transition from February 28 to March 1 of the leap year. The secular perpetual calendar, such as the one developed by IWC in the Portugieser Eternal Calendar, extends autonomy to 3999, also incorporating the correction of century years of the Gregorian calendar—years divisible by 100 but not by 400, such as 1900, which are not leap years.
From the workshop, the practical distinction is this: with an annual calendar you live well all year round, you set it once at the end of February, and the mechanism remains relatively compact compared to a perpetual. For those who don't want to think about it, the perpetual is the choice; for those who accept one adjustment per year in exchange for more accessible construction, the annual calendar is an honest compromise.