Definitions for "Perpetual calendar"
A calendar that can be used perpetually or over a wide range of years. That of Capt. Herschel covers, as given below, dates from 1750 to 1961 only, but is capable of indefinite extension.
Calendar that automatically accounts for the different lengths of the months as well as the leap years of the Gregorian calendar.
Set of calendars for finding the day of the week for a given date during a wide range of years