calendar-date-style is a variable defined in `calendar.el'.
Its value is

Your preferred style for writing dates.
The options are:
`american' - month/day/year
`european' - day/month/year
`iso' - year/month/day
This affects how dates written in your diary are interpreted.
It also affects date display, as well as those calendar and diary
functions that take a date as an argument, e.g. `diary-date', by
changing the order in which the arguments are interpreted.

Setting this variable directly does not take effect (if the
calendar package is already loaded). Rather, use either
M-x customize or the function `calendar-set-date-style'.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.1 of Emacs.