1. Divide the year for which you are trying to determine a leap year by four. If the year is not divisible by four, it is not a leap year. If the year is divisible by four, continue
http://www.ehow.com/how_7546950_calculateleapyea...

The modern civil/business calendar inserts February 29th in a leap year. Every year divisible by '4' is a leap year, except for the last year of each Century . 1800, 1900, 2000, etc
http://wiki.answers.com/Q/How_are_the_leap_years_c...

During a leap year, one extra day is added to the year during the month of February making 366 day of that year. Leap years occur every 4 years unless it is a century year, then it
http://www.ask.com/webanswers/Society/Other/how_o...

Rule 1: Any year evenly divisible by 400 IS a leap year. Rule 2: Any year evenly divisible by 100 is NOT a leap year, unless it conflicts with Rule 1. Rule 3: Any year evenly divisible
http://answers.yahoo.com/question/index?qid=200712...

A day on Earth does not really last 24 hours. It takes 23 hours, 56 minutes and 4.1 seconds for the Earth to make a complete rotation. Because of this, the Earth actually rotates about 365.24 times per year. Adding an extra day every four years helps to synchronize the calendar year with the solar year. If the discrepancy were exactly one quarter of a year, then every four years would be a leap year without variation. Since the difference is 0.24 instead of 0.25, one leap year is skipped three times out of every 400 years. Leap year originated in the 16th century, when people realized that the vernal equinox was falling on March 11 instead of March 21. Pope Gregory XIII adjusted the calendar by moving the date ahead by 11 days and instituted the practice of adding an extra day to the calendar every four years.