In 1582, ten days simply vanished from the calendar.
The Julian calendar ran about 11 minutes too long per year; by 1582 the spring equinox had drifted ten days. Pope Gregory XIII therefore decreed that Thursday, 4 October 1582 would be followed directly by Friday, 15 October. Protestant countries hesitated: Britain only switched in 1752 and by then had to drop eleven days.
Useful: When you compare pre-1752 dates from England and Italy, they can be ten days apart.
Source: en.wikipedia.org