[norwaypinesavage]

This also explains why George Washington has two birthdays. On the day he was born, the calendar said it was February 11. However, the accrued errors from the Julian calendar made the day to be February 22 when England switched to the Gregorian calendar. Now the geniuses in Washington DC declare it to be the third Monday in February, making it to be in error most of the time, and don't even call it George Washington's birthday any more.

[Oratam]

I remember reading in some old history books the notation (O/S) meaning “Old Style” for a given date in colonial history. I don’t think historians bother with it any more.