I recently changed the time format setting from Control Panel | Regional Options from "HH:mm:ss" with no am/pm suffix to "hh:mm" with the am/pm suffix. The Outlook calendar adjusted to the change, but Explorer only changed from 24-hour to 12-hour. It did not include the am/pm suffix. And, yes, the field is wide enough to show it.
Is there some other setting I have to make?