I think you'll need to use date_transform - something like this (but I can't remember all the codes for the dates and times - you'll need to look up the AM,PM one at least).
<%GT::Date::date_transform($fieldname,'%mmm% %d% %yyyy%, %hh%:%MM%','%mmm% %d% %yyyy%')%>
<%GT::Date::date_transform($fieldname,'%mmm% %d% %yyyy%, %hh%:%MM%','%mmm% %d% %yyyy%')%>