A calendar introduced by Pope Gregory XIII in the year 1582. The calendar is based upon the Julian calendar. Ten days from the Julian calendar were surpressed by the switch to the Gregorian calendar. The years consist of 365 days with 366 in Leap Years which occur in years that are devisible by four with the exception that years that are multiples of 100 are only Leap Years if they are also divisible by 400. The Gregorian calendar is the calendar that is still in common use today.