There is a great deal of confusion surrounding dates in English (and Russian) genealogy. This stems from two things: The use of 'Years of Reign' of the Kings and Queens; and the Julian and Gregorian calendars.
I will try to solve this confusion here.
Years of Reign
Throughout English genealogy you will find dates of events referred to as "1 Edw. IV" or "2 Hen. VIII" etc. These refer to the year of reign of that monarch, for example: 2 Hen. VIII is the year 1510 as King Henry VIII ascended the throne in 1509. For a complete listing of the reigns of the Kings and Queens of England and Great Britain click here.
In 46 BC Julius Ceasar set the length of the year at 365 days, and 366 days every fourth (Leap) year. There were thirty and thirty-one days in the months, alternately, except February which had twenty-nine in ordinary years and thirty in leap years. The month of July was named after him.
The Emperor Augustus spoilt this arrangement by naming August after himself and, not to be outdone by Julius Ceasar, he added 1 day to August and took away 1 day from February in both ordinary and leap years.
This calendar was not perfect as it was too short by 11 minutes and 14 seconds each year.
Pope Gregory XIII rectified this anomaly in 1582 when he decreed that the fifth of October should become the fifteenth, and that the centurial years should not be leap years unless they were divisible by 400.
England did not accept this calendar until 1752 when an Act of Parliament (made in 1750) made 3rd September 1752 into 14th September, and also moved the first day of the year to 1st January (previously 25th March).
Russia did not accept the Gregorian calendar until the 1917 Revolution.
This is the calendar we all use today.