If you have a date and a time such as 2010-08-24 18:00 you can first convert this to a (unix) time stamp with the strtotime command. You can then use that time stamp in the date command to convert it to whatever display format you need:
$timestamp = strtotime("2010-08-24 18:00");
$date_and_time = date("Y-m-d g:i A",$timestamp);
$just_time = date("g:i A",$timesatmp);