Why Do We Have Leap Years?

We need leap years to correct the calendar because the Earth's orbit isn't exactly 365 days long. It is 365 days six hours, eleven minutes and about 15 seconds. The difference which mounts up each year is corrected by adding an extra day at the end of February, every four years.
Even this is only a partial solution. Having a leap year brings the calendar back by 45 minutes. This gets corrected at the end of a century by missing out the leap year. It doesn't end there. This corrects the calendar by six hours too much. So to fix this every fourth end of century 'is' a leap year.
A leap year is always divisible by four unless it is the last year of the century. The last year of a century is a leap year if it is divisible by 400. Therefore 2000 was a leap year, but 1900 was not.