InterCal

InterCal is a date conversion program that offers a multitude of calendar systems, including Gregorian, Julian, Western Historical, Jewish, Islamic, French Revolutionary, Chinese, Mayan, Persian, Afghan, and Elliott Super. Although it's not an appointment calendar, it allows the user to display calendars, one month at a time, and jump around in time, from year to year or month to month.

This software is highly useful for genealogical or historical research, as well as for entertainment purposes. Its only limitation is the finite word size of the computer, with the exception of the Chinese calendar system, which can be found in the provided "Calendar System Facts" document.

Religious holidays can also be labeled in the month display, and all implemented systems allow the user to select any single day and see its equivalent in every other calendar system. Additionally, every day in the displayed month can have its equivalent in one other calendar system shown.

Overall, InterCal offers a diverse range of calendar systems and provides easy navigation through time, making it an excellent tool for anyone interested in exploring various calendar systems or conducting research.