Calendars and Timekeeping

ACalendar.com
The calendar has many options with a standard monthly interface, including sunrise, sunset, and Moon phases.
http://www.acalendar.com/

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

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

Astronomy and Space Calendar
Features different views of the night sky, constellations, stars, planets, and other celestial wonders.
http://www.newarkmuseum.org/PermanentCollection/PermanentPlanetariumCollection.aspx?id=1816

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

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

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

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

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://www.hermetic.ch/cal_stud.htm

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/

Calendopaedia
The encyclopaedia of calendars.
http://calendopedia.com/

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

The Chinese Calendar
The mathematics and rule explanation for the calendar.
http://www.math.nus.edu.sg/aslaksen/calendar/chinese.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

GMT: Greenwich Mean Time
Complete guide to times in every time zone in the world.
http://wwp.greenwichmeantime.com/

Hindu Calendar Dates 52nd Century
Explains the cosmological and astronomical influences in creating the Indian calendar.
http://senthilarumugam.tripod.com/id10.html

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

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

The Julian and Gregorian Calendars
Explanations of the Julian Calendar, the Gregorian Reform, the adoption of the Gregorian Calendar, the length of the tropical year and related matters.
http://www.hermetic.ch/cal_stud/cal_art.html

Julian Day Numbers
An explanation of the Julian day number system used by astronomers and calendricists.
http://www.hermetic.ch/cal_stud/jdn.htm

Leap Seconds
Civil time is occasionally adjusted by one second increments. A detailed explanation of what a second actually is, and why leap seconds are necessary.
http://tycho.usno.navy.mil/leapsec.html

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.lunarcal.org/

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

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

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

PlexGlobe: International Clock and Global Kit
A productivity toolkit for global users and businesses that includes world clock and calendar.
http://www.plexobject.com/software/plexglobe/

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

Time and Date
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/

Types of Calendars
An explanation of lunar, solar, lunisolar, rule-based and other calendars.
http://www.hermetic.ch/cal_stud/lunarcal/types.htm

World Holidays: Weeks and Months
Information on the origin of the week, and etymology of month and weekday names.
http://www.gkindia.com/holidays/weekdays.htm