- Leap Year
 - Brief discussion on the reason for leap years, including graphs. From Eric Weisstein's World of Astronomy. - Leap Years
- Find out how leap years came about, and how a certain year is determined to be a leap year. From the Royal Greenwich Observatory.
- timeanddate.com: Why Leap Years?
- Explains why leap years are used in modern calendars, the history behind the day, and how to calculate a leap year.
- Why Do We Have Leap Years?
- Explanation of the occurrence from Cecil Adams' The Straight Dope.
- Wikipedia: Leap Year
- Article describing the complex process in which a leap year is calculated using the Gregorian, Julian, Chinese, Hebrew, Hindu, and Iranian calendars.
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