Old Style vs. New Style years
Your 2/11/1731 O.S. date of birth for George Washington is off by one year. It should be 2/11/1732 O.S.
Our encyclopedia entry for George Washington gives his birthday as Feb. 22, 1732 in our current calendar, and Feb. 11, 1731, Old Style. At first glance, this seems absurd; while the Julian calendar that used to be used wandered off course over the centuries, the difference amounted to eleven days at that point, not a year and eleven days.
There's one more detail, however: under the calendar in use when Washington was born, the year began on March 25, not January 1. As a result, February 11, 1731 O.S. is, in fact, February 22, 1732 in our current system.
-The Fact Monster