Genealogists who research far enough back – and it’s not really that far in genealogical terms – will have to learn about the Julian and Gregorian calendars, and how each one affects dates that we recognize today.
“New Years Day” has not always been celebrated on the first of January. Calendars were developed to make sense of the natural cycles of time: days and years are derived from the solar cycle, while months come from the lunar cycle. There are many calendars, as it took some experimentation before we finally settled on the current system of leap years. For our purposes, we’re only concerned with the Julian and Gregorian calendars.
The Julian calendar resulted from Julius Caesar’s changes to conform more closely to the seasons within a year. Started in 45 BC, the Julian calendar held March 25th as the first day of the year, and every year was 365 days and 6 hours long. Pope Gregory XIII determined in 1582 the gradual problem that had developed when using the Julian calendar: over time the calendar was 10 days off the natural solar cycle. So Pope Gregory XIII created the Gregorian calendar, which is the one we use today. To compensate for the days which were out of sync, the Gregorian calendar dropped 10 days from October in 1582. The Pope also had everyone jump ahead by 10 days to make up for the lost days due to the earlier erroneous calendar.
Because of the calendar change in 1752 the calendar changed to a January 1st new year, and accommodations had to be made in the way dates were written to avoid confusion. Dates were designated O.S. (old style) and N.S. (new style). Such dates are usually identified by a slash mark [/] breaking the “Old Style” and “New Style” year, for example, March 19, 1631/2. Occasionally, writers would express the double date with a hyphen, for example, March 19, 1631-32. In general, double dating was more common in civil than church and ecclesiastical records.
Although current genealogical standards have us retain Old Style dates in transcriptions, historians and genealogists need to be aware that some people living at the time converted the date of an event, such as a birthday, from Old Style to New Style. George Washington, for example, was born on February 11, 1731 under the Julian Calendar, but changed the date to February 22, 1732 to reflect the Gregorian Calendar.
There are many pages online with more in depth explanations of the Julian and Gregorian Calendars. Simply search on Julian and Gregorian Calendars, or genealogy double dating.