GNU Emacs

31.13.5 Time Display Format

The calendar and diary by default display times of day in the conventional American style with the hours from 1 through 12, minutes, and either ‘ am’ or ‘ pm’. If you prefer the European style, also known in the US as military, in which the hours go from 00 to 23, you can alter the variable calendar-time-display-form. This variable is a list of expressions that can involve the variables 12-hours, 24-hours, and minutes, which are all numbers in string form, and am-pm and time-zone, which are both alphabetic strings. The default value is:

(12-hours ":" minutes am-pm
          (if time-zone " (") time-zone (if time-zone ")"))

Here is a value that provides European style times:

(24-hours ":" minutes
          (if time-zone " (") time-zone (if time-zone ")"))

Note that few calendar functions return a time of day (at present, only solar functions).