Hi Angus .In 1582 Pope Gregory XIII made adjustments to the leap year system (only centuries where the first two digits divided by 4 would be leap years). To adjust dates the following year, 1583 was cut by 10 days. Catholic countries immediately accepted this. One by one Protestant countries adopted the system (at different dates!); the last was England which in 1752 dropped 11 days to synchronize with the continent. The Notes for FTW confirm that this was also the changeover date in America. The Russians dropped 13 days in 1918. My book (Time-Life 1970) says the Russian Orthodox Church has still not accepted the system Beware that Jews and Moslems do not use the Gregorian Calendar Jim McArthur