Leap years
Note, the years 0 BC and 0 AD do not exist, they will be treated as 1 AD and 1 BC respectively
Note, these are done by the current calendar and assume that no different calendars have been used between the dates you specify
Rules: Leap years occur on years where the second two digits (AD only) are divisible by 4. In the event of the last two digits being 00, the first two digits must also be divisible by 4 for the year to be a leap year. For BC, counting starts at 1, so there is no year 0. This means that the leap years are offset by 1 and can be calculated by the same method as above, but with the year number increased by 1.