Definition of Leap Years

If the 4-digit year is not evenly divisible by 4, it is not a leap year.

If it is evenly divisible by 4 and also evenly divisible by 100, it is not a leap year — unless it is also evenly divisible by 400.

Therefore:

• If it is evenly divisible by 4 and not evenly divisible by 100, it is a leap year.
• If it is evenly divisible by 4 and is also evenly divisible by 400, it is a leap year.
• If it is evenly divisible by 4 and also evenly divisible by 100 and also evenly divisible by 400, it is a leap year.
• However, if it is evenly divisible by 4 and also evenly divisible by 100 and not further evenly divisible by 400, then it is not a leap year.
  

1600 was a leap year. 1700, 1800, and 1900 were not leap years. 2000 was a leap year.

From [http://www.willmaster.com/blog/perl/Calculate_Leap_Years.html]