I had a quick search on Google and it came up with this:
"The universally accepted date of 14 October 1066 is recorded using the Julian calendar, which was the calendar system in active use across Europe at the time.
Historians generally preserve original dates for events that occurred before the introduction of the Gregorian calendar to avoid historical confusion.
What would the date be today?
If you were to align the Battle of Hastings with our modern Gregorian calendar system (a process called using a proleptic Gregorian calendar), the date would shift forward by 6 days to 20 October 1066.
The calculation works as follows:
• In the 11th century, the Julian calendar lagged behind astronomical time by approximately 1 day every 128 years.
• By October 1066, that mathematical drift had accumulated to exactly 6 days.
• When Pope Gregory XIII introduced the modern calendar in 1582, the gap had reached 10 days, and by the time Great Britain adopted it in 1752, the discrepancy required skipping 11 days entirely."
