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The practice of double dating resulted from the switch from the Julian to the Gregorian calendar. Not all countries and people accepted this new calendar at the same time. England and the American colonies didn't officially accept it until 1752. Before that date, the government 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. (see table)

There was a discrepency of eleven days bu\y the time England and the American Colonies adopted the new calendar. 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 birth certificates, or other documents).

Julian calendar

The Julian calendar was introduced in 46 BC by Julius Caesar and came into force in 45 BC (709 ab urbe condita). It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year, known at least since Hipparchus. It has a regular year of 365 days divided into 12 months, and a leap day is added to February every four years. Hence the Julian year is on average 365.25 days long.

The notation "Old Style" (OS) is sometimes used to indicate a date in the Julian calendar, as opposed to "New Style" (NS), which indicates a date in the Gregorian calendar. This notation is used when there might otherwise be confusion about which date is found in a text.

Gregorian Calendar

Although the Julian calendar remained in use into the 20th century in some countries and is still used by many national Orthodox churches, it has generally been replaced for civil use by the modern Gregorian calendar. Orthodox Churches no longer using the Julian calendar typically use the Revised Julian calendar rather than the Gregorian calendar.

The Gregorian calendar is the most widely used calendar in the world. A modification of the Julian calendar, it was first proposed by the Calabrian doctor Aloysius Lilius, and was decreed by Pope Gregory XIII, for whom it was named, on 24 February 1582 via the papal bull Inter gravissimas. Years in the calendar are numbered from the traditional birth year of Jesus, which has been labeled the "anno Domini" (AD) era[1], and is sometimes labeled the "common era" or the "Christian Era" (CE).

The Gregorian Calendar was devised both because the lunar calendar had grown conspicuously wrong, since the Julian Calendar year is slightly too long, so that the vernal equinox slowly drifts backwards through Julian calendar years. This caused problems in computing the date of Easter.

The Gregorian calendar system dealt with these problems by dropping a certain number of days to bring the calendar back into synchronization with the seasons, and then slightly shortening the average number of days in a calendar year, by omitting three Julian leap-days every 400 years. The days omitted are in the century years (specifically: 29 February 1700, 1800, 1900; 2100, 2200, 2300; 2500, 2600, 2700; 2900, etc., are dropped).