<Pedantic Mode ON/>

Unfortunately you have to take into account the fact that the Gregorian Calendar was adopted by the UK on September 2 1752 (the very next day was September 14) Also note that this alteration was required to correct the total drift that the Julian calendar had caused since its adoption, hence if you went back 600 years just using the Gregorian definition the drift is another

* different *number of days - 4.5 given the average lengths of the years* (Gregorian years are a tiny tad shorter than Julian years)

* Okay, it's more complicated than that, we have 600 years where some are Julian, some are Gregorian....

<Pedantic Mode OFF/>