Variable: display-time-format
display-time-format is a customizable variable defined in time.el.gz.
Value
nil
Documentation
String specifying format for displaying the time in the mode line.
See the function format-time-string for an explanation of
how to write this string. If this is nil, the defaults
depend on display-time-day-and-date and display-time-24hr-format.
Source Code
;; Defined in /usr/src/emacs/lisp/time.el.gz
(defcustom display-time-format nil
"String specifying format for displaying the time in the mode line.
See the function `format-time-string' for an explanation of
how to write this string. If this is nil, the defaults
depend on `display-time-day-and-date' and `display-time-24hr-format'."
:type '(choice (const :tag "Default" nil)
string))