If you've done much genealogical
research, you may have noticed the practice of "double Dating."
In many early colonial American records, you will find entries
like "12 February 1732/33," or "14 January 1701/02."
The explanation for this seemingly strange practice is found
in the history books.
Many people at the time added eleven days to their birthdates so they might celebrate the exact anniversary. This explains why original records indicate that George Washington was born on 11 February, but that after 1752, his birthdate has always been considered to be 22 February. At this point, we must regretfully throw one last fly into the ointment. The original records indicate that George Washington was born on 11 February 1731. We recognize Washington's birthdate as 22 February 1732.
Now, we have already accounted for the eleven-day discrepancy. What about this business of an extra year?
For a long time, England and her colonies had observed two separate New Year's Days. The government and civil authorities considered 25 March to be New Year's Day and the people at large considered 1 January to be the beginning of the new year.
Needless to say, there was quite a bit of confusion. In an attempt to clarify things, many people took to using both years in their notations. For example, you will find dates similar to Washington's birthday written 1731/32. Or, you may find the same information expressed as 1731 O.S. (old Style), or 1732 N.S. (new style). At any rate, along with reconciling solar and lunar time in 1752, the English government also officially recognized 1 January as New Year's Day.
You should only encounter double dating of years in or prior to 1752. For the reasons given above, you will only encounter double dating of date (e.g. 1/12 February 3/14 March) in the months January, February and March in or prior to 1752.
from How To Trace Your Family Tree by the American Genealogical Research Institute Staff