A perpetual calendar is an advanced complication in watchmaking, this automatically takes account of the varying lengths of months and leap years to display the correct date, not requiring any corrections by hand. In this sophisticated mechanism lies the recognition of the Gregorian calendar's intricacies and smooth transition into a month of variable numbers of days and correctly identifying February 29th during leap years.
The perpetual calendar shows both the date and other additional information, like the day of the week, month, and in some cases, even the moon phase. This demonstrates the height of mechanical watch engineering, as it is at the same time practical and a manifestation of the watchmaker's competence in producing highly functional and reliable timepieces.