- ACalendar.com - The calendar has many options with a standard monthly interface, including sunrise, sunset, and Moon phases.
- Annus Novus Decimal Calendar - A proposal for a non-sectarian, culturally neutral calendar system.
- Astro Portal - CalSKY - The worldwide interactive online astronomical/space calendar and calculator - for friends of astronomy, as well as astronomers.
- Astronomical Time Keeping - Extensive descriptions of many timing systems, with information on time zones and Julian day numbers.
- Astronomy and Space Calendar - Features different views of the night sky, constellations, stars, planets, and other celestial wonders.
- Astronomy by JavaScript: Sun Calculators and More - Applets related to calendars and keeping time, including sidererial time.
- Blue Moon Myths - Second full moon in a month, or third full moon in a single season.
- Britannica.com - Clockworks: From Sundials to the Atomic Second - Britannica.com explores the history of timekeeping, from sundials to cesium atomic clocks.
- 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.
- Calendars through the Ages - History and FAQs of calendars, from ancient Rome to outer space. Including Julian, Gregorian, Jewish, Islamic, Chinese, and Mayan.
- Calendopaedia - The encyclopaedia of calendars.
- Calendrical Calculations - Published by Cambridge University Press. Gives a unified algorithmic presentation of the Gregorian, ISO, Julian, Coptic, Ethiopic, and Islamic civil calendars.
- GMT: Greenwich Mean Time - Complete guide to times in every time zone in the world.
- Hindu Calendar Dates 52nd Century - Explains the cosmological and astronomical influences in creating the Indian calendar.
- Indian Moons - A list of many American Indian tribe's names for months, days, and other calendar related information.
- Julian Day Numbers - An explanation of the Julian day number system used by astronomers and calendricists.
- 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.
- 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.
- 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.
- 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.
- Millennium Mistake - Explains the difference between millennium and year 2000 as the absence of a year zero.
- PlexGlobe: International Clock and Global Kit - A productivity toolkit for global users and businesses that includes world clock and calendar.
- Rosetta Calendar - An online handy tool for converting to and from the Gregorian, Julian and Jewish calendars.
- The Calendar, Leap Years and the Year 2000 - An explanation of the calendar including the origin of the day, week, month and year.
- The Chinese Calendar - The mathematics and rule explanation for the calendar.
- The Difference Between the Millennium and Year 2000 - Questions and answers to satisfy the hearts of true millennium buffs.
- The Islamic Calendar - The mathematics of the calendar in Singapore.
- 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.
- 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.
- Time and Dates - Information on date, time, and time zones from the New York Public Library.
- Types of Calendars - An explanation of lunar, solar, lunisolar, rule-based and other calendars.
- World Holidays: Weeks and Months - Information on the origin of the week, and etymology of month and weekday names.
