English people of that time started the new year on March 25, Annunciation Day (or Lady Day). Any dates between January 1, and March 25, should have one year added. In 1650 they started saying "1649-50" for that period of the year.
Between 1600 and 1700 the date should be carried forward ten days.
Between 1700 and 1752 the date should be carried forward eleven days.
In 1752 the English Parliament made September 3rd into September 14th, and the New Year to start on January 1st, thereby correcting the calendar for the current time.