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

((month "[-/]" day "[^-/0-9]") (year "[-/]" month "[-/]" day "[^0-9]") (monthname " *" day "[^-0-9]") (year " *" monthname " *" day "[^0-9]") (dayname "\\W"))

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

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

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.3 of Emacs.