It is the difference between
Julian to
Gregorian calendars. The British empire converted to Gregorian in 1752. This moved New Years from 25 March to 1 January.
From Wikipedia...
Quote:
1751 in England only lasted from 25 March to 31 December. The dates 1 January to 24 March which would have concluded 1751 under the old calendar became part of 1752 when the beginning of the numbered year was changed from 25 March to 1 January.
People born before 25 March on the Julian calendar who retroactively adopted a Gregorian birthday saw themselves born "a year later" than they were under the old calendar. This also depends on the offset days between the two calendars, which 11 days at the time of adoption by the British empire.
So in this example, Frederick was born 1 February 1707 under the Gregorian calendar and 21 January 1706 under the Julian calendar.