Astronomy: Calendars and Timekeeping

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.

Extensive descriptions of many astronomical time keeping systems, with information on time zones and Julian day numbers.

Lots of time and date related information, such as yearly and monthly calendars, counters, countdown, and the world clock which shows the current time in cities all over the world.

Information on date, time, and time zones from the New York Public Library.

Second full moon in a month, or third full moon in a single season?

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.

Britannica.com explores the history of timekeeping, from sundials to cesium atomic clocks.

Articles on the Gregorian and Julian calendars, the ISO date format, the Julian day number system, the Maya calendar, the Goddess lunar calendar, the Liberalia Triday Calendar and C functions for date conversion; plus software for calendrical conversion.

LunarCal is a 160-year perpetual Chinese Lunar Calendar for 1900 to 2060. Chinese festivals are listed and the moon phase is displayed.

The mathematics of the Chinese calendar. Explains the rules for the Chinese calendar.

The mathematics of the Islamic calendar in Singapore.

Questions and answers to satisfy the hearts of true millennium buffs.

History and FAQs of calendars, from ancient Rome to outer space. Including Julian, Gregorian, Jewish, Islamic, Chinese, and Mayan.

A proposal for a non-sectarian, culturally neutral calendar system.

Applets related to calendars and keeping time, including sidererial time.

Published by Cambridge University Press. Gives a unified algorithmic presentation of the Gregorian, ISO, Julian, Coptic, Ethiopic, and Islamic civil calendars.