Certain dates appear in this record as double years, e.g. March 1, 1739/1740. The background of this is that prior to 1752 Great Britain and her colonies used the Julian Calendar. This calendar considered that the length of a solar year was 365.75 days and allowed a leap year (extra day) every fourth year. The problem with the Julian Calendar was that a solar year is actually 365.2422 days. In other words the leap year overcorrected. This error accumulated and events such as the vernal equinox (March 21) fell on incorrect dates.
In 1582 Pope Gregory XVIII issued a proclamation deleting ten days to correct the accumulated error and also providing that three leap years every four hundred years would no longer be added. The new calendar was called the Gregorian Calendar and had the additional feature of changing New Years Day. Under the older Julian Calendar New Years day was March 25th (the Annunciation). Under the old calendar December 31, 1750 would be followed by January 1, 1750 and March 24, 1750 would be followed by March 25, 1751. In order to prevent confusion these dates between January 1st and March 24th are often written or transcribed as double years. December 31, 1750 is followed by January 1, 1750/51 and March 25, 1751 is preceded by March 24, 1750/51. The remaining dates of the year are unaffected and written with a single year.
Pope Gregory's calendar reform was adopted immediately by the Catholic powers of Europe. However, at the time Europe was divided by the Reformation and the protestant states did not wish to appear too quick to adopt a reform promulgated by Rome. Great Britain waited until 1752 and feeling that a respectable period of time had past finally adopted the reform.
The calendar change created all kinds of problems. A major issue is calculating an age in years, months and days. A person who was born under the Julian Calendar and died under the Gregorian Calendar; for example, George Washington; lived for an extra eleven days that the dates of birth and death under the particular calendar in use at the time did not reflect. Some records will use the terms "Old Style" (O.S.) and "New Style" (N.S.) to indicate which calendar date was used.
