The Gregorian Calendar was adopted in 1582.  Pope Gregory XIII authorized this “New Style” calendar.  As part of the change, October became shorter by 10 days, and the definition of leap year changed.  January 1 was the first day of the year.  Protestant countries, including England and its colonies, did not recognize the Pope’s authority, so they continued using the old Julian Calendar.

Because of this change, more than one calendar was in use in Europe and European colonies.  And, in England there were 2 starts of the year.  The “legal” year began on March 25.  However, also January 1 was used as the start of the New Year.

Dates that fell between these 2 dates for the “new year” were interpreted using a slash mark [/] between the 2 dates.  An example is March 2, 1635/36.

In 1752, England and its colonies changed to the Gregorian calendar.  The beginning of the “legal” year began January 1, and September 1752 was shortened by 11 days.

Now you know what it means when you see a slash in the date!