The Emacs calendar displayed is always the Gregorian calendar, sometimes called the "new style" calendar, which is used in most of the world today. However, this calendar did not exist before the sixteenth century and was not widely used before the eighteenth century; it did not fully displace the Julian calendar and gain universal acceptance until the early twentieth century. This poses a problem for the Emacs calendar: you can ask for the calendar of any month after January, 1 AD, but the calendar displayed will be the Gregorian, even for a date at which the Gregorian calendar did not exist!
Emacs knows about several different calendars, though, not just the Gregorian calendar. The following commands describe the date indicated by point in various calendar notations:
You can find out the date on the ISO (International Standardization Organization) commercial calendar--used largely in Europe--with the C command. The ISO date equivalent to the date indicated by point will be displayed in the echo area.
If you need to know the equivalent date on the Julian ("old style") calendar, use the J command. This will display in the echo area the Julian date for the day point is on. The Julian calendar is no longer in common use.
If you want to know the equivalent date on the Hebrew (Jewish) calendar, use the H command; this will display in the echo area the Hebrew date for the day point is on. The Hebrew calendar is the one used to determine the dates of Jewish holidays.
To find the equivalent date on the Islamic (Moslem) calendar, use the I command; this will display in the echo area the Islamic date for the day point is on. The Islamic calendar is the one used to determine the dates of Moslem holidays. Note that there is no universal agreement in the Islamic world about the calendar; Emacs uses a widely accepted version, but the precise dates of Islamic holidays often depend on proclamation by religious authorities, not on calculations. As a consequence, the actual dates of occurrence can vary slightly from the dates computed by Emacs.
To find the equivalent date on the French Revolutionary calendar, use the F command; this will display in the echo area the French Revolutionary date for the day point is on. The French Revolutionary calendar is no longer in use.
You can move to dates that you specify on the Commercial, Julian, Hebrew, or Islamic calendars:
These commands will ask you for a date on the other calendar, move the point to the date equivalent to that date, and display the other calendar's date in the echo area.