The Calendar Change
Old Style vs. New Style
      Reform of the calendar took place in 1582.  At that time the old Julian calendar was modified to conform more closely to the actual length of the year and seasonal changes.   The new calendar, known as the Gregorian, started the year on January 1, where it begins today, instead of March 25 where it began in the Julian calendar.
     Although most other countries adopted the new Gregorian calendar, England continued to use the Julian calendar for almost 200 years; it wasn't until 1752 that England - and its colonies - finally switched over to the new system. This can be troublesome when one is dealing with dates during that period.
     The difficulty stems from the fact that, before 1752, most dates occurring during the first quarter of the year had to be considered two ways to be accurate. For example, George Washington's birth, February 22, occured in 1731 by the Julian calendar, for the Julian year 1731 did not end until March 25. Consequently the date in those days would be written February 22 1731-2 (1731 by the old style of reckoning and 1732 by the new). The two forms are usually abbreviated: O.S. (old style) and N.S. (new style).
     Genealogists have to keep these circumstances in mind when dealing with dates in the first quarter of the year before 1752 in this country and England. For the colonial United States were then provinces of the British Crown, and, with the exception of such interruptions as the Dutch periods of possession of New York State (The Dutch had adopted the new calendar), the British calendar was the old Julian calendar. Without using the observance and attention to detail of  a birth, death or marriage written by one method could be a year off with a recording by the other.

Martha Taylor Dumas Dec 15, 2011
As you probably know, early Quakers did not use the traditional names of the months (because of their derivations from the names of “pagan” gods), but  referred to them by numbers instead.  In the Old Style calendar, “First Month” would have referred to March rather than to January.
Martha Taylor Dumas
(claiming descendancy from Thomas Pearsall and Sarah Underhill)