Pope Gregory XIII's new calendar took effect on Friday, October 15, 1582, and has worked brilliantly every day ever since.

These reforms were embodied in the Gregorian Calendar, the most accurate calendar ever devised, erring by only 1 day every 3,323 years. Catholic countries quickly obeyed and instituted the reforms. Protestants either abhorred or completely ignored them, but eventually admitted their validity and belatedly adopted them. Great Britain and the thirteen American colonies did not implement them until 1752 when Parliament decreed that Wednesday, September 2, 1752, was to be followed by Thursday, September 14, 1752. Russia did not adopt the Gregorian Calendar until the Russian Revolution of 1917. Ironically, the Russian Orthodox there had vehemently rejected it, whereas the Catholic Church’s most fanatical atheist enemies, the Communist Bolsheviks, warmly embraced it. Today the Gregorian Calendar is widely used throughout the world in almost all countries, except the Islamic ones.