Calendar function Lange models incorporate calendar displays as either annual calendars or perpetual calendars. An annual calendar combines the calendar functions for the date, day of the week and month. The annual calendar automatically adjusts for differing month durations of 30 and 31 days. If the calendar of a mechanical watch is labelled ‘perpetual’, it also adjusts for leap years and possesses a leap year display.
Our calendar displays are based on the Gregorian calendar. It is classed as one of the solar calendars as it uses the position of the sun. The average length of a year under it is 365.2425 days. As a result, it comes very close to the average solar year, which has a length of 365.21219 days.