The modern calculations are all specified according to the Gregorian calendar so an allowance of 6 days had to be made when calculating the tides. However, all the dates referred to in the literature and reported here, are Julian dates.

We ‘lost’ 10 days in 1582 when the Gregorian calendar was adopted. The calendars were coincident in 325 AD. So the ‘day loss’ varies from 10 to 0 days over that period according to the formula that adjusts the date by inserting ‘leap centuries’. In 1066 the difference between the Julian and Gregorian dates was only 6 days, and so 20 September 1066 in the Julian calendar became 26 September 1066 in the Gregorian calendar.

The date of the battle is specified in the written sources using the calendar of saint’s days. I failed to do the necessary conversion in a way that agreed with the received day or date. However, I defer to the antiquarians who have given us the date of 20^th September 1066 because this is very close to my own attempts at converting dates. If subsequent work by an expert in ecclesiastical calculation was to show that the received wisdom was wrong by a day or two, I would not demur. The impact of any possible adjustments does not have a significant impact on the tidal calculations.