Until the time of Julius Caesar the Roman year was organized round the phases of the moon. For many reasons this was hopelessly inaccurate so, on the advice of his astronomers, Julius instituted a calendar centred round the sun. It was decreed that one year was to consist of three hundred and sixty-five and a quarter days, divided into twelve months; the month of Quirinus was renamed ‘July’ to commemorate the Julian reform.
Unfortunately, despite the introduction of leap years, the Julian calendar overestimated the length of the year by eleven minutes fifteen seconds, which comes to one day every on hundred and twenty-eight years. By the sixteenth century the calendar was ten days out. In 1582 reforms instituted by Pope Gregory XIII lopped the eleven minutes fifteen seconds off the length of a year and deleted the spare ten days. This new Gregorian calendar was adopted throughout Catholic Europe.
Protestant Europe was not going to be told what day it was by the Pope, so it kept to the old Julian calendar. This meant that London was a full ten days ahead of Paris. By the time England came round to adopting the Gregorian calendar, in the middle of the eighteenth century, England was eleven days ahead of the Continent.