"The 1582 reform of the calendar under Pope Gregory XIII produced--what else?--the Gregorian calendar, which replaced the Julian calendar (named after Julius Caesar, who instituted it). Catholic countries soon adopted Gregory's reform, but Protestant countries rejected it. They didn't want anything to do with a 'popish calendar.' Eventually they had to give in (astronomy eventually trumps prejudice), with Britain adopting the calendar only in 1752. There it was called the New Style calendar so no mention need be made of a pope. The Julian calendar was termed the Old Style."
[Tip of the hat to Karl Keating, E-Letter of 5/30/06]