Strange thing is, the computer is not set that way and the calendar's appointments show 12 hour time except when I hover the mouse over the appt. This is Outlook 2013 with Exchange/ Office 365.

I recommend that you post this in the Outlook Forum:  http://social.technet.microsoft.com/Forums/en-US/outlook/threads

Hi,

Please start Outlook in safe mode to check whether the issue persists. If the issue doesn't happen in safe mode, it may be caused by some third-party add-ins. We can enable the add-ins which is disabled in safe mode one by one to find out the main cause. For more information, please refer to Step 6: Start Outlook in safe mode in the following KB to check whether the issue is caused by the temporary changes that occur when you start Outlook in safe mode:

http://support.microsoft.com/kb/2934538/en-gb

Regards,