What is the purpose of the Gregorian/Julian radio button?
The Gregorian calendar is the calendar in common use today. It differs from the Julian calendar which preceded it by the rule for determining leap years. Specifically, under the Julian calendar a leap year occurs in every year that is divisble by 4, whereas in the Gregorian calendar the leap year does not occur in years divisible by 100 unless they are also divisible by 400. In addition to changing the rules for leap years, the introduction of the Gregorian also caused a change in the calculation for the Paschal Full Moon. The switch from the Julian calendar to the Gregorian calendar occurred at different times in different places, ranging from 1582 for most of Catholic Europe to 1929 in China. The Gregorian/Julian radio button will change the calendar dates as well as change the algorithm for computing the Paschal Full Moon. It should be noted that you can set the button to Gregorian and enter a year preceding 1582. In that case the dates shown are what the Gregorian dates would have
