- 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. - 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.
- Leap Year
- Brief discussion on the reason for leap years, including graphs. From Eric Weisstein's World of Astronomy.
- Why Do We Have Leap Years?
- Explanation of the occurrence from Cecil Adams' The Straight Dope.
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