Calendar user settings contain the option to change the time format of the calendar between 24 and 12h formats. If this is not set, the the language default is picked up.
There isn't an easy way to use set the time format for the whole site (unlike start of the week and similar).
This patch adds a new setting which replicates the user setting for the site. If set to default is ignored, or can force 12/24hr format.