You can customize the way dates are displayed in the diary, mode
lines, and messages by setting calendar-date-display-form.
This variable holds a list of expressions that can involve the variables
month, day, and year, which are all numbers in
string form, and monthname and dayname, which are both
alphabetic strings. In the American style, the default value of this
list is as follows:
((if dayname (concat dayname ", ")) monthname " " day ", " year)
while in the European style this value is the default:
((if dayname (concat dayname ", ")) day " " monthname " " year)
The default ISO date representation is:
((format "%s-%.2d-%.2d" year (string-to-number month)
(string-to-number day)))
Another typical American format is:
(month "/" day "/" (substring year -2))