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"20 March 1727 (OS 20 March 1726; NS 31 March 1727)"
This seems to me not to be a Julian Calendar vs Gregorian Calendar issue: based on my experience, differences between the Julian Calendar and the Gregorian Calendar dates are between the number of 11–13 days, not 365 days?
Both sites you link to give the same date (in the current Gregorian calendar). The other date mentioned in Wikipedia is the "old style" (Julian calendar) date.
Specifically, en.wikipedia.org/wiki/Isaac_Newton has footnote a explaining this Old Style dating problem, which you flatly deny? If you doubt that narrative from WP, it is up to you to explain why "this is not a Julian Calendar vs Gregorian Calendar issue"?
@JaneB. As the note on the Wikipedia page says, "he died in the period after the start of the New Style year on 1 January, but before that of the Old Style new year on 25 March. His death occurred on 20 March 1726 according to the Old Style calendar, but the year is usually adjusted to 1727. A full conversion to New Style gives the date 31 March 1727"
@LаngLаngС,SteveBird: So technically it's correct that this isn't due to a confusion between the Julian & Gregorian calendars. The issue is that the change from "New Style" to "Old Style" in England also involved redefining which day started the year.
When Newton died, the British started the new year on March 25 (Lady Day). In 1752, the UK switched to starting the New Year on January 1. Dates between January 1 and March 25 prior to 1752 happened in different years Old Style vs. New Style.
