An annual calendarAnnual Calendar An annual calendar is a complication in a mechanical watch that automatically adjusts for the different lengths of the months, except February, and only requires one manual adjustment per year at the end of February. It has a mechanism that takes into account the different number of days in each month and automatically adjusts the date accordingly. [Learn More] is a complication in a mechanical watch that automatically adjusts for the different lengths of the months, except February, and only requires one manual adjustment per year at the end of February. It has a mechanism that takes into account the different number of days in each month and automatically adjusts the date accordingly.