I think it gets it from your system.
Under Windows, you need to go to Control Panel -> Regional settings and set the time formats there.
Not sure about MAC
You are on the right track. In the Organizer window showing all the thumbnails the time format is correct. But if I want to Adjust Date and Time and change to a specified date and time (it is a picture scanned some time ago) the time required is PM or AM.
It is Windows 7.