the difference between the Julian calendar and what the Gregorian calendar

The difference between the Gregorian and Julian calendars added about 3 days every 400 years. The difference was about 11 days during Greg 16's papacy and St. Bigfoot was about 1000 years earlier, so, if my rough mental adding machine isn't slipping too many gears it was about 4 days different during St. Biggie's era.