In 45 B.C., most of the world began using the Julian calendar, designed by Julius Caesar, 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, Gregory XIII created the Gregorian calendar. This is the calendar that we use officially in the United States. The new calendar changed the first day of the year from March 25 to January 1. Gregory also had everyone jump ahead by 10 days to make up for the days that were lost with 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.

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 old birth records or birth certificates). Also, since the birth of the Internet, many "documents" (e-copies) have been posted by people unaware of the calendar differences, who assumed the "double-year" dates were errors and simply picked one or the other. . . . . Beware!

As for day and month, old English/American documents dated before 1752 may need adjusting to account for calendar changes, i.e., The Mayflower Compact, but adjustments can't be done arbitrarily by adding 10 or 11 days. Libraries and history departments of universities have researched old documents and are the best sources for correctly adjusted dates.