A precise C library to provide arithmentic for the most common calendar systems. Currently Gregorian, Julian, Milankovic, Solar Hijri (also known as Shamsi or Jalali), Islamic Civil and Jewish (also know as Hebrew) calendar systems are provided.

The easiest way to install libcalendars is to use PPA:

sudo add-apt-repository ppa:soroush-r/solap
sudo apt update
sudo apt install libcalendars1    # installs the library
sudo apt install libcalendars-dev # Installs development headers

Alternatively you can compile the package from source:

mkdir build && cd build
cmake ..
cmake --build . --config Release
sudo cmake --install . --config Release

In case you also want to build and run unit tests:

cmake -DBUILD_TESTING=ON ..
cmake --build . --config Release
ctest . --config Release

If nothing goes wrong, you should see all test passed:

    Start 1: Gregorian
1/6 Test #1: Gregorian ........................   Passed    0.31 sec
    Start 2: Julian
2/6 Test #2: Julian ...........................   Passed    0.59 sec
    Start 3: Milankovic
3/6 Test #3: Milankovic .......................   Passed    0.78 sec
    Start 4: SolarHijri
4/6 Test #4: SolarHijri .......................   Passed    0.89 sec
    Start 5: Jewish
5/6 Test #5: Jewish ...........................   Passed   17.46 sec
    Start 6: IslamicCivil
6/6 Test #6: IslamicCivil .....................   Passed    0.53 sec

100% tests passed, 0 tests failed out of 6

• Vala: vlibcal

Almost all algorithms in libcalendar are implemented using Julian Day calculations. You can convert any date on supported calendars to JDN and vice versa. For example:

uint32_t jdn = 0;
sh_to_jdn(&jdn, 1392, 04, 15);
printf("Julian Day for 1392/04/15 AP is: %l\n", jdn);

Which prints:

$ Julian Day for 1392/04/15 AP is: 2456480

You can also use non-jdn broken-down date API. For example you can check if a year in Solar Hijri calendar is leap or not:

if(sh_is_leap(1395)) /* returns 0 for regular years and 1 for leap years */
    printf("Yep\n");

You can also convert calendar dates to/from Gregorian calendar:

int16_t y;
uint8_t m;
uint16_t d;
sh_to_gr(1396, 06, 20, &y, &m, &d);
printf("1396/06/20 AP is %04d-%02d-%02d\n", y, m, d);
gr_to_sh(2017, 09, 11, &y, &m, &d); 
printf("2017-09-11 is %04d-%02d-%02d AP\n", y, m, d);

which will print:

1396/06/20 AP is 2017-09-11
2017-09-11 is 1396/06/18 AP 

Reference of API is available at https://soroush.github.io/libcalendars.

This library is written in the hope that it will be useful. With your help, libcalendars can be better (: You can help libcalendars in several ways:

1. Build the library and thest its output. You can raise an issue if you've found any problem.
2. Contribute to improve code quality, fix bugs and add new features. Please read our Code of Conduct for more details.

This library is implemented in C programming language, using no external dependecies. The C standard library used in libcalendar is C11. Though it should be possible to compile this library with a C99 compiler.

Most of the conversion algorithms for JDN to calendar and vice versa are implemented based Dr Louis Strous's work (available online on Astronomy page). Namely Gregorian, Julian, Milankovic and Islamic Civil calendars and their JDN calculations are adopted from above page. Solar Hijri (Shamsi) and Jalali calendar calculations are implemented based on Dr. Mousa Akrami's work on median year length for Persian calendar. (See notes on Solar Hijri calendar).

Following is a list of supported calendars, and a short description (mostly from wikipedia) about them.

The Gregorian calendar is internationally the most widely used civil calendar. It is named after Pope Gregory XIII, who introduced it in October 1582.

The calendar was a refinement to the Julian calendar[3] involving a 0.002% correction in the length of the year. The motivation for the reform was to stop the drift of the calendar with respect to the equinoxes and solstices—particularly the northern vernal equinox, which helps set the date for Easter. Transition to the Gregorian calendar would restore the holiday to the time of the year in which it was celebrated when introduced by the early Church. The reform was adopted initially by the Catholic countries of Europe. Protestants and Eastern Orthodox countries continued to use the traditional Julian calendar and adopted the Gregorian reform after a time, at least for civil purposes and for the sake of convenience in international trade. The last European country to adopt the reform was Greece, in 1923. Many (but not all) countries that have traditionally used the Islamic and other religious calendars have come to adopt this calendar for civil purposes.

The Julian calendar, proposed by Julius Caesar in 46 BC (708 AUC), was a reform of the Roman calendar. It took effect on 1 January 45 BC (AUC 709), by edict. It was the predominant calendar in the Roman world, most of Europe, and in European settlements in the Americas and elsewhere, until it was refined and gradually replaced by the Gregorian calendar, promulgated in 1582 by Pope Gregory XIII. The Julian calendar gains against the mean tropical year at the rate of one day in 128 years. For the Gregorian the figure is one day in 3,030 years. The difference in the average length of the year between Julian (365.25 days) and Gregorian (365.2425 days) is 0.002%.

The Revised Julian calendar, also known as the Milanković calendar, or, less formally, new calendar, is a calendar, developed and proposed by the Serbian scientist Milutin Milanković in 1923, which effectively discontinued the 340 years of divergence between the naming of dates sanctioned by those Eastern Orthodox churches adopting it and the Gregorian calendar that has come to predominate worldwide. This calendar was intended to replace the ecclesiastical calendar based on the Julian calendar hitherto in use by all of the Eastern Orthodox Church. The Revised Julian calendar temporarily aligned its dates with the Gregorian calendar proclaimed in 1582 by Pope Gregory XIII for adoption by the Christian world. The calendar has been adopted by the Orthodox churches of Constantinople, Albania, Alexandria, Antioch, Bulgaria, Cyprus, Greece, Poland, and Romania.

The Solar Hijri calendar, also called the Solar Hejri calendar or Shamsi Hijri calendar, and abbreviated as SH, is the official calendar of Iran and Afghanistan. It begins on the vernal equinox (Nowruz) as determined by astronomical calculation for the Iran Standard Time meridian (52.5°E or GMT+3.5h). This determination of starting moment is more accurate than the Gregorian calendar for predicting the date of the vernal equinox, because it uses astronomical observations rather than mathematical rules.

Each of the twelve months corresponds with a zodiac sign. The first six months have 31 days, the next five have 30 days, and the last month has 29 days in usual years but 30 days in leap years. The New Year's Day always falls on the March equinox.

My implementation of Solar Hijri (Shamsi) calendar is based on median year calculation obtained from Muousa Akrami's work: The development of Iranian calendar: historical and astronomical foundations - 2014. This method is more accurate than 33-year algorithm and supports a wider range of dates, both in Solar Hijri <-> Gregorian comversions, and in JDN calculations.

The Islamic, Muslim, or Hijri calendar is a lunar calendar consisting of 12 months in a year of 354 or 355 days. It is used (often alongside the Gregorian calendar) to date events in many Muslim countries. It is also used by Muslims to determine the proper days of Islamic holidays and rituals, such as the annual period of fasting and the proper time for the pilgrimage to Mecca.

The Islamic calendar employs the Hijri era whose epoch was retrospectively established as the Islamic New Year of AD 622. During that year, Muhammad and his followers migrated from Mecca to Yathrib (now Medina) and established the first Muslim community (ummah), an event commemorated as the Hijra. In the West, dates in this era are usually denoted AH (Latin: Anno Hegirae, "in the year of the Hijra") in parallel with the Christian (AD) and Jewish eras (AM). In Muslim countries, it is also sometimes denoted as H from its Arabic form. In English, years prior to the Hijra are reckoned as BH ("Before the Hijra").