Civil time is occasionally adjusted by one second increments called leap seconds. A detailed explanation of what a second actually is, and why leap seconds are necessary. The mathematics of the Chinese calendar. Explains the rules for the Chinese calendar. A list of many American Indian tribe's names for months, days, and other calendar related information.
Islamic months begin at sunset on the day of visual sighting of the lunar crescent.
This site discusses the concept of the International Lunar Date Line and contains numerous maps of crescent visibility. The calendar was the center of Maya life and their greatest cultural achievement. To carry out their astronomical calculations they developed a sophisticated mathematical system. The Maya Calendar website is developing with the intent of providing a complete view of Maya culture. The mathematics of the Islamic calendar in Singapore.