If it's a year ending in 00 where the first two digits are divisible by four (1600, 2000, etc.), it's a leap year. If it's not, it's not. Actually, little tiny adjustments (leap seconds) are made to the calendar as required. There are people who are way better at this sort of thing than I--I just study history, not time. Every rearrangement of the calendar since the creation of the Julian calendar (we now use the Gregorian one, as I recall) has been to line up the calendar and the year better.
"Now everyone was giving her that kind of look UFOlogists get when they suddenly say, 'Hey, if you shade your eyes you can see it is just a flock of geese after all.'"
"You can't erase icing."
"I can't believe it doesn't work! I found it on the internet, man!"