Because I am using GNOME Shell, the following may not work:
In the "Top bar" section of the "Tweak tool" (in "System settings"), you can choose whether the date is displayed; In "Date & time" (in "System settings"), a radio button allows to choose between "24 hours" or "AM/PM"; In the second tab of "Language support" (in "System settings"), you can choose to display the date in the usual format for "français (France)" (for instance).
