You can specify the way hours are displayed with the Hour Display setting on the Properties window. The default setting is 12 Hour, which displays the hours from 1:00 to 12:00, with an "a.m." or "p.m." extension. You can use the 24-Hour setting to display the hours in 24-hour time. An appointment that goes from 11:30 am to 1:00 pm with the 12-Hour setting will go from 1130 to 1300 with the 24-Hour setting.
To change the hour display, click SELECT on the setting you want and then click SELECT on the Apply button. The change takes effect immediately.