Old style dating
To restore proper synchronization with the seasons, the new calendar dropped 10 days from the month of October in 1582, and to prevent the problem of extra days from occurring again, 1 day was added to February for every year divisible by four (leap year).
Pope Gregory XIII introduced the Gregorian calendar on 24 February 1582, but the first countries to use the new calendar did not adopt it until 15 October 1582.
Over the next several years, more and more countries converted to the Gregorian calendar with Turkey being the last in 1926.
After continued resistance against adopting a Catholic invention, Britain finally converted to the Gregorian calendar on 14 September 1752, so starting with this date, dual dating no longer applied to Britain and its colonies.From the period of 15 October 1582 (the official start of the Gregorian calendar) to 13 September 1752 (the day before the start of Britains use of the calendar), Britain and its colonies used a system of dual dating to represent the dates of both calendars.Dual dating was a means of documenting dates using both the Julian and Gregorian calendars.In 45 BC, Julius Caesar brought about the Julian calendar as a reform of the Roman calendar.
In fact, there is a lot of misinformation on the Web that serves only to confuse the subject even further.
To clear up any confusion regarding the dating conventions used, I have attempted to explain, in simple terms, the concept of dual dating as it applies to historical documentation.