Resigned 8 April 1763 

It is not disputed that Bute's successor, Grenville, took office as PM on 16 April 1763.

But the 1911 Encyclopedia Brittanica gives Bute's date of resignation as 8 April 1763.   And a calculation from the dates and figures provided by 10 Downing Street (Total time as PM: 317 days) also indicates he resigned office 8 April.  And Marjie Bloy agrees on 8 April 1763.

I'm about to edit the dates in office.  If anyone reverts, please provide credible references.