The adjustment known as leap year isn't as simple as "every four years." If that's all we did, our calendar would still be quite screwed up. To be even more precise and keep the earth from gaining or losing too much time, every year that is a multiple of 100 but not a multiple of 400 will not be a leap year. You may remember that 2000 was a leap year, though 2100, 2200, and 2300 will not be leap years, and then 2400 will be one.
Want to experience them with me? Eat your vegetables.