Date Display Format
===================
You can customize the manner of displaying dates in the diary, in
mode lines, and in 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 ISO standard date representation is this:
(year "-" month "-" day)
This specifies a typical American format:
(month "/" day "/" (substring year -2))