... and most people don't realize that knowing that correct calculation is only half the battle. Once you know it, you can successfully navigate the Gregorian calendar. But what happens when you need to work with dates prior to the start of the Gregorian calendar? Does your Gregorian start happen in 1582 when the first countries adopted it, or in 1752 when the British adopted it? Most people simply apply Gregorian rules indefinitely into the past, which is not always correct for every situation:

http://en.wikipedia.org/wiki/Proleptic_Gregorian_calendar