The following commands describe the selected date (the date at point) in various other calendar systems:
calendar-print-iso-date).
calendar-print-julian-date).
calendar-print-astro-day-number).
calendar-print-hebrew-date).
calendar-print-islamic-date).
calendar-print-french-date).
calendar-print-chinese-date).
calendar-print-coptic-date).
calendar-print-ethiopic-date).
calendar-print-persian-date).
calendar-print-mayan-date).
If you are using X, the easiest way to translate a date into other calendars is to click on it with Button2, then choose Other Calendars from the menu that appears. This displays the equivalent forms of the date in all the calendars Emacs understands, in the form of a menu. (Choosing an alternative from this menu doesn't actually do anything—the menu is used only for display.)
Put point on the desired date of the Gregorian calendar, then type the appropriate keys. The p is a mnemonic for “print” since Emacs “prints” the equivalent date in the echo area.