calpak, a MATLAB code which computes various simple calendrical quantities. It can work with various calendars including Egyptian, English, French Revolutionary, Gregorian, Julian, and Julian Ephemeris Date. It can convert a date from one calendar to another. It can return the day of the week for a given date. It can convert from day-number/year to day/month/year format. It can calculate the time difference between two dates.

Some common methods of marking the date include:

While there have been many calendars over the years, it is instructive to contemplate just the crazy story of our current "common" calendar. To this day, people disagree about whether there was a year 0, although the Julian calendar was a Roman invention, and Dionysius Exiguus, who gets the blame for shifting the Julian calendar's starting date to the birth year of Christ four hundred years afterwards, didn't have an accurate idea of when that was.

There was a controversial shift from the Julian to the Gregorian calendar, which took place piecemeal throughout the Catholic world, with several countries actually switching back and forth more than once, and with England holding out on the old system until after George Washington was born (which means he was born on February 11 AND February 22).

People didn't agree on when the year started, so that January and February, in particular, were a little murky about which year they belonged to, and the year sometimes started around March 22, near the vernal equinox.

There are a number of side issues, including


The computer code and data files described and made available on this web page are distributed under the MIT license


calpak is available in a C version and a C++ version and a FORTRAN90 version and a MATLAB version and a Python version.

Related Data and Programs:


calendar_nyt, a MATLAB code which shows the correspondence between dates and the New York Times volume and issue number;

dates, a dataset directory which contains lists of dates in various calendar systems.

doomsday, a MATLAB code which is given the year, month and day of a date, and uses John Conway's doomsday algorithm to determine the corresponding day of the week.

weekday, a MATLAB code which determines the day of the week for a given day.

weekday_zeller, a MATLAB code which uses Zeller's congruence to determine the day of the week corresponding to a given date, such as 13 July 1989, Gregorian calendar, ... which was a Thursday.


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Source Code:

Last revised on 12 March 2021.