We’ll, that’s interesting:

During Newton’s lifetime, two calendars were in use in Europe: the Julian (“Old Style” calendar in Protestant and Orthodox regions, including Britain; and the Gregorian (“New Style”) calendar in Roman Catholic Europe. At Newton’s birth, Gregorian dates were ten days ahead of Julian dates; thus, his birth is recorded as taking place on 25 December 1642 Old Style, but it can be converted to a New Style (modern) date of 4 January 1643. By the time of his death, the difference between the calendars had increased to eleven days. Moreover the civil or legal year in England began on 25 March, therefore the Newton’s death on 20 March was still dated as 1726 O.S. there.

Source: en.wikipedia.org/wiki/Isaac_Newton

Me and my mother were looking into this because of a random discussion about dating systems and her time in the Navy; an interesting thing about the Julian calendar is that one of its date forms (ordinal) is still in somewhat-common use by the US military; just with the Gregorian leap days added in. So today’s date could be rendered as 25359 (2025 day #359).