Why historical events can have two dates - a quick explanation!
Do you know your Julian from your Gregorian calendar? No? Well let me explain.
Sometimes you can see two dates given for an historical event and that's because the number of days in our calendar changed! Prior to 1582 the Julian calendar had been used throughout the known Western world. The problem with the Julian Calendar was that it did not align to the realities of the Earth's movement around the Sun. Put simply, there were more days in the calendar year than it took the Earth to go around the Sun - 1 more every 128 years to be exact. In 1582 Pope Gregory XIII decreed, via a Papal Bull, that 10 days be dropped from the Roman Calendar, and the Gregorian Calendar was created.
However, not all countries adopted the calendar at the same time and, as the difference between the 2 calendars increases over time, the number of days dropped in the calendar of each country depended on when they decided to move over. The Gregorian calendar was adopted in England and America in 1752 by dropping 11 days. Turkey was the last country to officially move over, on 1 January 1927 and had to drop 13 days in order to align. The Gregorian Calendar also has an error rate but of only 1 day in 3236 years. The picture below shows the Calendar for September 1752.
Written by Philippa Brewell for British History Tours
Further reading (Accessible site): http://www.thetimenow.com/blog/the-roman-calendar/