my $hash={};
while (have_records){
       my $time = (date_from_record) . "-" . (time_from_record);
       if ( ! defined($hash->{$time} ) {
             $hash->{$time} = {   min => SOME_VERY_LARGE_NUMBER,
                                  max => 0 
                               }
        }

        $hash->{$time}->{min} = ( (VALUE_FROM_RECORD) < $hash->{$time}->{min} ? 
                                  (VALUE_FROM_RECORD) : $hash->{$time}->{min} ) ;

        $hash->{$time}->{max} = ( (VALUE_FROM_RECORD) > $hash->{$time}->{max} ? 
                                  (VALUE_FROM_RECORD) : $hash->{$time}->{max} ) ;
}

# NOTE:  change (date_from_record), (have_records and other 
# psuedo-values to things that are reasonable.