Leap Seconds
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.
http://tycho.usno.navy.mil/leapsec.html

Astronomical Time Keeping
Extensive descriptions of many astronomical time keeping systems, with information on time zones and Julian day numbers.
http://www.maa.mhn.de/Scholar/times.html

Time and Date
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.
http://www.timeanddate.com/

Time and Dates
Information on date, time, and time zones from the New York Public Library.
http://www.circ.uab.edu/nypldr/1time/index.htm

Blue Moon Myths
Second full moon in a month, or third full moon in a single season?
http://www.infoplease.com/spot/bluemoon1.html

Maya Calendar 2000
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.
http://mayacalendar.com

Britannica.com - Clockworks: From Sundials to the Atomic Second
Britannica.com explores the history of timekeeping, from sundials to cesium atomic clocks.
http://www.britannica.com/clockworks

Calendar Studies
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.
http://hermetic.nofadz.com/cal_stud.htm

LunarCal - A Perpetual Chinese Lunar Calendar
LunarCal is a 160-year perpetual Chinese Lunar Calendar for 1900 to 2060. Chinese festivals are listed and the moon phase is displayed.
http://www.geocities.com/lunarcal

The Chinese Calendar
The mathematics of the Chinese calendar. Explains the rules for the Chinese calendar.
http://www.math.nus.edu.sg/aslaksen/calendar/chinese.shtml

The Islamic Calendar
The mathematics of the Islamic calendar in Singapore.
http://www.math.nus.edu.sg/aslaksen/calendar/islamic.shtml

The Difference Between the Millennium and Year 2000
Questions and answers to satisfy the hearts of true millennium buffs.
http://www.timeanddate.com/counters/mil2000.html

Calendars through the Ages
History and FAQs of calendars, from ancient Rome to outer space. Including Julian, Gregorian, Jewish, Islamic, Chinese, and Mayan.
http://webexhibits.org/calendars/

Annus Novus Decimal Calendar
A proposal for a non-sectarian, culturally neutral calendar system.
http://www.atlantium.org/calendaran.html

Astronomy by JavaScript: Sun Calculators and More
Applets related to calendars and keeping time, including sidererial time.
http://www.jgiesen.de/astro/astroJS/

Calendrical Calculations
Published by Cambridge University Press. Gives a unified algorithmic presentation of the Gregorian, ISO, Julian, Coptic, Ethiopic, and Islamic civil calendars.
http://emr.cs.uiuc.edu/home/reingold/calendar-book/index.shtml

Calendopaedia
The Encyclopaedia of Calendars.
http://www.geocities.com/calendopaedia/

Millennium Mistake
Explains the difference between millenium and year 2000 as the absence of a year zero.
http://www.millenniummistake.net/

Rosetta Calendar
An online handy tool for converting to and from the Gregorian, Julian and Jewish calendars.
http://rosettacalendar.com/

The Calendar, Leap Years and the Year 2000
An explanation of the calendar including the origin of the day, week, month and year.
http://chemistry.csudh.edu/oliver/smt310-handouts/calendar.htm

Indian Moons
A list of many American Indian tribe's names for months, days, and other calendar related information.
http://www.americanindian.net/moons.html

Astro Portal - CalSKY
The worldwide interactive online astronomical/space calendar and calculator - for friends of astronomy, as well as astronomers.
http://www.calsky.com/

Lunisolar Calendar
Lunar calendar with lunations, from new moon to new moon, instead of months. Moon phase on each day. Chinese, Jewish, and Islamic months. Eclipses, meteors, planets, star charts. Christian, Pagan, Islamic, Hindu, and Jewish holidays.
http://www.pburch.net/lunarcal.html