Geek Calendar Tool
This program calculates and displays calendar-related information. For a given date, the various information about different calendar systems is available:
- The date in the Gregorian calendar. This is the currently used calendar by most of the world, and was first decreed by Pope Gregory XIII in February 1582, replacing the older Julian Calendar.
- The date of the next new and full moons. Also the current lunar phase. These are estimated, but usually to within one day.
- For any given year, the Golden Number and the Epact. These are numbers related to the cycles of the Sun and the Moon. They are used to calculate the date of Easter, and were also used in olden times to predict solar eclipses.
- The dates of the Paschal full moon and Easter Sunday. The Paschal full moon is the first full moon after the vernal equinox (21 March). Easter Sunday is defined as the next Sunday after that. Note, the Paschal full moon does not necessarily correspond to the astronomical event.
- The Julian Day Number. This number is sometimes used in astronomy, and uniquely indentifies every day since 1 January 4713 BC.
- The date in the Julian Calendar. This older calendar was introduced by Julius Caesar in 45 BC. It remained in use in Britain (and dominions) until September 1752. It was then replaced by the Gregorian calendar due to the latter's more accurate leap year system.
- The date in the Mayan Calendar. The ancient Mesoamericans developed a calendar which used three
different dating systems in parallel. These are (1) the "Long Count" - a system of five digits "A.B.C.D.E" which
counted days since the start of the Mayan era; (2) the "Tzolkin" (divine calendar) which described each day by a
named part and a digit which cycle every 260 days; (3) the "Haab" (civil calendar) which has a month/day
arrangement which cycles every 365 days. In this respect the Haab is similar to the modern Western calendar.
- The approximate date in the Islamic Calendar. This cannot be calculated precisely as it depends on lunar observation - the program estimates the date with a typical error of one or two days.
- The date in the Hebrew (Jewish) Calendar. This uses a lunisolar cycle, where each year may contain either 12 or 13 months. It is still used to determine the dates for Jewish religious holidays.
- The Chinese year. The start date of the Chinese year cannot be easily be calculated precisely by an algorithm, but is generally in January or February. The previous and next Chinese new year dates are shown if known, together with information on the current Chinese year.
- The French Revolutionary Calendar. This calendar was introduced in France on 24 Nov 1793, but began counting dates from 22 Sep 1792. There were 12 months of 30 days each, with 5 or 6 additional days to match the 365/366 of the solar year.
- The number of days between two dates. If a reference date is supplied, the program will calculate the number of days between the reference and the current date. You can use this to find out how many days old you are, or to count down to a future date.
Dates are written in International Standard ISO-8601 "year-month-day" format. No guarantees are made regarding accuracy or correctness of this software.
- Claus Tondering's Calendar FAQ is an excellent source of calendar-related information.
- SourceForge zmanim project - Jewish Calendar Calculation.
- Bryan Derksen - Maya Numerals.
Geek Clock Tool
Daylight World Map
4 Mar 2020
Geek Calendar Tool V1.40 released - Added Coptic (Egyptian) calendar.
Clock related information: The decimal, hex, and binary clocks have been improved and moved into a separate app, Geek Clock Tool.
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