for($date){
   ($h,$m,$s)=split/:/;
   s/\:\d\d$//;  #throw out the seconds, we don't need them
   $m="00" if ($m<15);
   $m="15" if (($m>=15) && ($m<30));
   $m="30" if (($m>=30) && ($m<45));
   $m="45" if ($m>=45);
   s/\:\d\d\/\:$m/; #replace the minutes with the generalized value
}