diary-european-date-forms

diary-european-date-forms is a variable defined in `calendar.el'.
Its value is

((day "/" month "[^/0-9]")
 (day "/" month "/" year "[^0-9]")
 (backup day " *" monthname "\\W+\\<\\([^*0-9]\\|\\([0-9]+[:aApP]\\)\\)")
 (day " *" monthname " *" year "[^0-9]")
 (dayname "\\W"))

• This variable may be risky if used as a file-local variable.

Documentation:
List of pseudo-patterns describing the European style of dates.
The defaults are: DAY/MONTH; DAY/MONTH/YEAR; DAY MONTHNAME;
DAY MONTHNAME YEAR; DAYNAME. Normally you should not customize this, but
`diary-date-forms' (which see).

You can customize this variable.