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Calendrical software

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Some calendars are sufficiently simple that their use requires no more calculation than can be done mentally. An example is the Gregorian Calendar, whose use requires nothing more complicated arithmetically than division of a year number by 400 (even though the rule for leap years is sufficiently complex that many people cannot understand it without mental effort).

To determine the structure (of years, months, days, etc.) of a calendar it may be necessary to use complex calculations. An example is the Chinese Lunar Calendar, which requires an exact determination of the time of the dark moon at a particular longitude. The Chinese Calendar was invented over 2000 years ago, long before there were any computers in the modern sense of the term, so clearly a computationally intensive calendar can be developed and used without reliance on modern computers. But these days computation is indispensible in the study of calendars.

This computation is of two kinds: That required simply to compute the structure of a calendar, and that required for more scholarly studies (for example, the extent to which new years day in a solar calendar varies with respect to one of the seasonal markers, in particular, the vernal equinox). This article discusses calendrical software of the first kind only.

The obvious way to classify calendrical software of this kind is in terms of the calendars that it deals with. Some software, of course, handles dates in more than one calendar, so in what follows there is some duplication.

In each of the sections below a distinction is made between software which is written for the PC, the Macintosh, some other system, or whether the software is a web application. Some of this software is free and some is try-before-you-buy (the name of the software publisher is usually given after the title).

Software listed under "DOS (Windows console applications)" was originally written for MS-DOS and either (i) can be run under Windows by double-clicking on the program icon as usual or (ii) can be run under Windows but only by invoking the MS-DOS Prompt.

There are a couple of additional sections not concerned with specific calendars, such as astronomical software of relevance to calendrical studies.

Annus Novus Decimal Calendar[]

Windows software

This software (by Hermetic Systems) is for converting dates in the Common Era (a.k.a. Gregorian) Calendar to and from dates in the Annus Novus Decimal Calendar.

Arithmetic Operations on Calendar Dates[]

The basic operations are the addition or subtraction of a number of days to or from a date, and the subtraction of one date from another (resulting in the number of days between two dates). Some software allows the option of including or excluding the first date, and of excluding weekend days from the calculations.

Windows software

Web applications

Armenian Calendar[]

DOS (Windows console applications)

Astronomical Software[]

Windows software

Web applications

DOS (Windows console applications)

  • SOLEX Aldo Vitagliano. - Computer program modelling the N-body dynamics of the Solar System.

Aztec Calendar[]

Web applications

DOS (Windows console applications)

  • Pohualli 1.01 Arnold Lebeuf and Arkadiusz Soltysiak. - A Computer Simulation of the Mesoamerican Calendar System.

Bahá'í Calendar[]

Windows software

  • The Kalendis Calendar Calculator (freeware) Irv Bromberg. Western, Future Astronomical, and experimental leap day, leap week, and leap month "Bahá'í-inspired" variants.

Web applications

Chinese Lunar Calendar[]

The Chinese calendar is a lunisolar calendar whose months begin on the day of the dark moon, and whose years usually have 12 months but sometimes 13 (approximately every third year) .

Windows software

K H Leong's LunarCal program (up to v6.24) has incorrect Qing Ming date for years 2009, 2013, 2017, etc.

Web applications

Macintosh software

Common Era Calendar (Gregorian Calendar)[]

All (or almost all) calendar software intended for users in the West convert dates in some calendar to or from CE dates. Only the following kinds of software might be expected not to support conversion of CE dates: (a) Software intended for local use in a non-Western society. (b) Software for machine control lacking any way for humans to interact with it.

Coptic Calendar[]

Web applications

DOS (Windows console applications)

Dee Calendar[]

Windows software

  • The Julian-Gregorian-Dee Date Calculator (Hermetic Systems) is for converting between Julian, Gregorian, Dee and Dee-Cecil calendar dates, for adding and subtracting a number of days, weeks, months and years to or from a given date, and for calculating vernal equinoxes.

Web applications

Easter date[]

Web applications

DOS (Windows console applications)

Ecliptic Calendar[]

Web applications

Ethiopic Calendar[]

Web applications

Extraterrestrial Calendars[]

Web applications

French Revolutionary Calendar[]

Windows software

Web applications

Macintosh software

DOS (Windows console applications)

Goddess Calendar[]

Windows software

  • The McKenna-Meyer Goddess Calendar, an accurate 13-month lunar calendar in which months coincide with the lunar cycles. It is a modification of a calendar originally proposed by Terence McKenna in 1987.

Web applications

Hebrew Calendar[]

Windows software

Web applications

DOS (Windows console applications)

Hermetic Leap Week Calendar[]

The Hermetic Leap Week Calendar has years which consist of exactly 52 or 53 7-day weeks, and 12 months each of 4 or 5 weeks. The northern winter solstice almost always occurs in the last week of the year.

Windows software

Web applications

Hermetic Lunar Week Calendar[]

The Hermetic Lunar Week Calendar specifies days by means of dates of the form year-month-week-day, where a year has 12 or 13 months, a month has exactly four weeks, and a week consists usually of 7 or 8 days but sometimes of 6 or 9 days. Weeks track the lunar quarters exactly.

Windows software

Web applications

Icelandic Calendar[]

Web applications

Indian Calendars[]

Web applications

Islamic (Hijri) Calendar[]

Windows software

Web applications

DOS (Windows console applications)

ISO Week Calendar[]

The ISO Week Calendar is a leap week calendar that is used mainly in government and business for fiscal years. It uses the conventional 7-day week, with weeks starting on Monday. Years begin within 3 days of the start of the Common Era Calendar year, and each year has exactly 52 or 53 weeks.

Windows software

Web applications

Japanese Calendar[]

Windows software

  • Chinese Calendrics Hermetic Systems (The 'reference longitude' must be changed to 'Japan'.)

Web applications

Julian Calendar[]

The Julian Calendar was introduced by Julius Caesar in the year 46 BCE. In 1582 CE it was reformed to produce the Gregorian calendar. It is still used by the Russian Orthodox Church.

Windows software

Web applications

DOS (Windows console applications)

Julian Day Number[]

The Julian day number system is not itself a calendar, but is of central importance in the study of calendars. This system sets up a one-to-one correspondence between days (more exactly nychthemerons) as observed on Earth and the integers: ..., -2, -1, 0, 1, 2, 3 ... This allows identification of a particular day without reference to any particular calendar. Most conversions of dates in two different calendars do this by converting dates in those calendars to or from Julian day numbers.

Windows software

Web applications

DOS (Windows console applications)

Kalacakra Calendar[]

The Kalacakra Calendar is a lunisolar calendarin which days are numbered according to the Moon's longitude.

DOS (Windows console applications)

Kurdish Calendar[]

Web applications

Liberalia Triday Calendar[]

The Liberalia Triday Calendar combines a solar calendar with a lunar calendar which have 'tridays' (periods of 3 days) in common. The Solar Liberalia Triday Calendar divides years into four quarters and the Lunar Liberalia Triday Calendar divides years into twelve months.

Windows software


Maya Calendar[]

The Maya used at least three different calendrical systems (and some variations within the systems). The three systems are known as the tzolkin, the haab and the long count. The tzolkin is a cycle of 260 days and the haab is a cycle of 365 days. Since the Maya calendar is fairly complex, the need for software to handle dates in this calendar has long been recognized.

There is still some uncertainty regarding how dates in the Maya Calendar should be correlated with dates in the Common Era Calendar (this is known as the correlation problem), although there are good reasons to hold that 13.0.0.0.0 4 Ahau 3 Kankin corresponds to December 21, 2012. Thus all Maya Calendar software which converts Maya dates into European dates should have some way to set the so-called correlation number.

Windows software

Web applications

DOS (Windows console applications)

  • Mayan Calendrics Hermetic Systems - A freeware Maya Calendar date conversion program which allows specification of correlation number, year-bearer system and month base number.
  • Pohualli 1.01 Arnold Lebeuf and Arkadiusz Soltysiak. - A Computer Simulation of the Mesoamerican Calendar System.

Mac OS X applications

Meyer-Palmen Solilunar Calendar[]

The Meyer-Palmen Solilunar Calendar has dates in the form cycle-year-month-day where each cycle consists of 60 years. Most years have twelve months, but some have thirteen.

Windows software

Miscellaneous calculators[]

Windows software

Web applications

Julian/Gregorian/FrenchRepublican/Swedish,RevisedJulian in Civil Preference/
Roman date/Byzantine date/Coptic,Ethiopian in Coptic Preference/Jewish/Armenian/
Seleucid date,Babylonian date in Hellenistic Preference/Tabular,EphemerisBased in Islamic Preference/
SolarHijiri,SolarHijiriAlgorithmic,Borji,Jalali in Iranian Preference/
Shahanshahi,Fasli,Qadimi in Zoroastrian Preference/Bahai/
Surya siddanta,Modern LuniSolar in HinduLuniSolar Preference/
Surya siddanta,Modern Solar in HinduSolar Preference and Iranian Preference/
IndianNationalSolar,Nanakshahi,RevisedBengali in IndianGregorian Preference/
Tibetan in Tibetan Preference/ThaiLuniSolar in ThaiLuniSolar Preference/
Pranatamangsa/JavaneseLunar/BalineseLuniSolar/Tenganan/Yi/ChineseLuniSolar/ChineseSolar/
Eastasian Eras(Chinese,Japanese,Ryukyu,Korean,Manchurian,Rouran,Gaochang,Yunnan,WesternXia,Vietnamese)/
LongCount in Mayan Preference/Discordian/Darian in Darian Preference/Shire in Shire Preference/
World,Positivist,InternationalFixed,Tranquility,WorldSeason in OutOfWeek Preference/
Sym454,Sym010,Pax,HankeHenry in LeapWeek Preference/
DeeCecil,Dee,VanishingLeprechaun in LeapRule Preference/
YermLunar in Yerm Preference/GoddessLunar/ISO Week

Macintosh software

  • Takwim Ian Proudfoot. A downloadable web application, converts Western dates to Javanese and Muslim dates.

Nepali calendar[]

Windows software

Ordinal (Day-in-Year) Dates[]

An ordinal date is the number of days since the start of the year (usually the Common Era year), with Day 1 = the first day of the year.

Windows software

Web applications

Persian (Jalali) Calendar[]

Windows software

Web applications

DOS (Windows console applications)

Solstices and Equinoxes[]

Windows software

Web applications

  • Seasons Institut de Mécanique Celeste et de Calcul des Ephemerides

Macintosh software

Symmetry454 Calendar[]

Windows software

  • The Kalendis Calendar Calculator (freeware) Irv Bromberg. Including a wide range of experimental and demonstration variants of this calendar as well as the "Classical" Symmetry calendar.

Tabot Calendar[]

The Tabot Calendar is a Rastafarian calendar related to the Gregorian calendar.

Web applications

Today's date in the Tabot CalendarHermetic Systems

Tibetan Calendar[]

There is no Tibetan Calendar software at present which converts between Tibetan and Western dates because the exact calculational principles used by the Tibetan calendar makers from year to year has not been made public. There is, however, a web application for converting between Tibetan and Western years.

Windows software

Web applications

DOS (Windows console applications)

Turkish Financial Calendar (Mali- or Rumi-Calendar)[]

DOS (Windows console applications)

Vietnamese lunar calendar[]

Windows software

  • Chinese Calendrics Hermetic Systems (The 'reference longitude' must be changed to 'Vietnam'.)

Web applications

World Calendar[]

Web applications

World Dual Petin-Meton Calendar[]

Windows software

This software (by Hermetic Systems) is for converting between dates in the Lunar Petin-Meton Calendar, the Solar Petin-Gregorian Calendar and the Common Era Calendar.

Yerm Lunar Calendar[]

The Yerm Lunar Calendar is a simple rule-based pure lunar calendar, which gains accuracy by allowing its lunar year (the yerm) to be much longer than a solar year.

Web applications

Zoroastrian Calendar[]

Web applications

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