This is not being presented to explain a method of courting, but a method of record keeping. The following article has been adapted by me. Unfortunately, I have no record of the author for attribution. However, I believe it is important for researchers to fully understand double-dating for better genealogical record-keeping.
To understand why it was necessary, one must understand how time is measured. Most of the early calendars were based on phases of the moon. This lunar calendar consisted of a 12-month year of 29-1/2 days each.
The Julian calendar was used throughout the Middle Ages in Europe. Because of the half days, that calendar had a built-in inaccuracy of about three days in every four centuries.
In 1582 Pope Gregory XIII sponsored the Gregorian Calendar, which is the calendar in use today. By the year 1582, the Julian calendar was ahead of actual time by ten days. The Roman Catholic countries adopted the Gregorian Calendar in that year. However, since the new calendar was sponsored by a Pope, Great Britain and her colonies delayed acceptance until the passage of an Act of Parliament in 1751. By that time, the old calendar was off eleven days.
The Act provided that in 1752, the second day of September should be followed by the fourteenth day of September. In other words, exactly eleven days were dropped out of that year. In order to prevent the error from recurring, there is an addition of a day to the calendar each fourth year (Leap Year). To make the calendar more precisely match "sun time," no Leap Year days are added except in a century divisible by 400. Thus 1800 and 1900 were not Leap Years, but the year 2000 will be.
One other change was made in 1752, and that was the date of beginning the New Year. Various people in various ages have celebrated different New Year's Days. Some ancient races ended the year with a Harvest Festival, and the Jews still retain that season. Others began with the Vernal Equinox, and since Easter fell near that season, the date used for the religious New Year's Day by Christians was March 25th.
It is very important to know that during colonial days, January 1 was the beginning of the legal year and March 25, that of the religious new year.
Before 1752, there was likely to be confusion with regard to dates between 1 January and 25 March, unless we know what New Year's Day the person recording the event was using.
Careful recorders used a double date, which eliminated the uncertainty. Some others used the designation "O.S." to indicate "Old Style." George Washington was born 12 Feb. 1731/32, which means that the year was still 1731 if the New Year was reckoned as not beginning until 25 March, but that the year was already 1732 if it had begun 1 January.
Therefore, records were marked with double dates, where known, for all dates between January 1 and March 24 up to the year 1753. Starting in 1753, all calendars started January 1 and only one year designation was necessary.