Beginning in 45 B.C., many parts of the world used the Julian calendar to mark the passage of time. According to the Julian calendar, March 25 was the first day of the year and each year was 365 days and 6 hours long. In 1582, Pope Gregory XIII determined that the Julian calendar was incorrect: each day was just a little bit too long. This meant that the human calendar wasn’t keeping up with nature’s calendar, and the seasons kept arriving slightly earlier in the year. To solve the problem, Pope Gregory XIII created the Gregorian calendar. This is the calendar that we use officially in the United States. As you know, this new calendar changed the first day of the year from March 25 to January 1. Pope Gregory also had everyone jump ahead by 10 days to make up for the days that were lost when the world was using the old Julian calendar.
The practice of writing double dates resulted from this switch from the Julian to the Gregorian calendar, and also from the fact that not all countries and people accepted the new calendar at the same time. For example, England and the American colonies didn’t officially accept the new calendar until 1752. Before 1752, the English government still observed March 25 as the first of the year, but most of the population observed January 1 as the first of the year. For this reason, many people wrote dates falling between January 1 and March 25 with both years, as in the following examples.
Julian or Old Style Gregorian or New Style Double Date
December 25, 1718 December 25, 1718 December 25, 1718
January 1, 1718 January 1, 1719 January 1, 1718/19
February 2, 1718 February 2, 1719 February 2, 1718/19
March 25, 1719 March 25, 1719 March 25, 1719
By the time England and the colonies adopted the new Gregorian calendar, the discrepancy between the two calendars was eleven days, instead of ten. To resolve the discrepancy, the government ordered that September 2, 1752 be followed by September 14, 1752. Some people also added 11 days to their birth dates (a fact which is not noted on their birth certificates).
The above explanation appears courtesy of Family Tree Maker Software.by