Can you set AM/PM in a GregorianCalendar date or is it something you can only get

Calendar.get(Calendar.HOUR);

Gives the hour (0-12) for AM/PM format.

Calendar.get(Calendar.HOUR_OF_DAY);

Gives the hour ranging from 0-24.

It does the conversion on its own. You don't need to tell it.

cal.set( Calendar.AM_PM, Calendar.AM )

Will/Could change the point time this calendar object represents. (If it's 1:00PM you will have 1:00AM afterwards). This is true for GregorianCalendar (see comment to question by Peter Cetinski).

Btw/Hint imo you should use a DateFormat to ouput your preferred format.

Calendar cal = new GreogorianCalendar();   
cal.set( Calendar.AM_PM, Calendar.AM );