calendar-iso-date-display-form is a variable defined in `calendar.el'.
Its value is

((format "%s-%.2d-%.2d" year (string-to-number month) (string-to-number day)))

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

Pseudo-pattern governing the way a date appears in the ISO style.
Normally you should not customize this, but `calendar-date-display-form'
(which see).

You can customize this variable.

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