The papal bull of February 1582 decreed that 10 days should be dropped from October 1582 so that 15 October should follow immediately after 4 October, and from then on the reformed calendar should be used.

Most protestant countries adopted the calendar between 1699-1701. Great Britain, Ireland and the American colonies, did not switch over until 1752 – by which time it was necessary to cut 11 days from the calendar, instead of 10, to bring the British calendar into line. Wednesday September 2nd 1752 was immediately followed by Thursday September 14th 1752.

After 1752, most major Western countries were using the Gregorian calendar. Some smaller countries took much longer to go over to using it. Romania did not switch to it until 1919. Turkey did not use it until 1927.

Changes in the 1500s required 10 days to be dropped. Changes in the 1600s required 10 days to be dropped. Changes in the 1700s required 11 days to be dropped. Changes in the 1800s required 12 days to be dropped. Changes in the 1900s required 13 days to be dropped. For example, when Soviet Russia undertook its calendar reform in February 1918, they moved from the Julian calendar to the Gregorian. This move resulted in a loss of 13 days, so that 1 February 1918, became 14 February.