Datetime t = Datetime.now();
System.debug(t);
System.debug(t.format('HH hh kk KK'));
Result:
13:37:57:233 USER_DEBUG [16]|DEBUG|2013-05-17 11:37:58
13:37:57:234 USER_DEBUG [17]|DEBUG|04 04 04 04
H,h,k,K all returned 04, but I'm pretty sure it should be 11 instead!