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 XII 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- the calendar that we use officially in the United States- with the first day of the year set to January 1. He also had everyone jump ahead by 10 days to make up for the days lost by using the Julian calendar.

The practice of writing double dates resulted from the switch to the Gregorian calendar, and also from the fact that not all countries 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. For this reason, many people wrote dates falling between January 1 and March 25 with both years, as in the following examples:

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).