CALENDAR_RD, a C++ program which returns information about a given date, by Edward Reingold, Nachum Dershowitz.
The program is interactive. The user enters y, m and d, which represent the year, (numeric) month, and day of a given date. Note that y must be positive. Also note that all dates are implicitly using the Gregorian calendar, extended backwards indefinitely.
Given date = (y,m,d), the program returns
The program halts when a nonpositive value of y is entered.
This code is in the public domain, but any use of it should publicly acknowledge its source.
CALENDAR_RD is available in a C++ version.
ANALEMMA, a C++ program which evaluates the equation of time, a formula for the difference between the uniform 24 hour day and the actual position of the sun, by Brian Tung.
CALPAK, a C++ library which converts between dates as computed on various calendars.
DATES, a dataset directory which contains lists of dates in various calendar systems.
DOOMSDAY, a C++ library 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 C++ library which determines the day of the week for a given day.
Edward Reingold, Nachum Dershowitz.