Old Style

Old Style, often written O.S., marks a date given by the Julian calendar. In England, Wales, Ireland and the American colonies it was the legal calendar until 1752, and the legal year began on March 25.

In American genealogy the label is used in two ways: for a date that has not been shifted by the 11 days dropped in 1752, and loosely for a year that started on March 25. When you copy a date from before 1752, keep it as written and mark it O.S. rather than silently converting it.