for ( date2offset($start) .. date2offset($end) ) {
      push @{$effective[$_]}, \$data;
  }

##</code><code>##

  my $results = $effective[date2offset($date)];	# O(1)
  if ( defined($results) {
      # @$results is an array of all matching $data
  }

##</code><code>##

  sub date2offset {
      my($yyyymmdd) = @_;
      my $year = int($yyyymmdd/ 10000) - 1970;
      my $mmdd = $yyyymmdd % 10000;
      my $month = int($mmmdd / 100) - 1;
      my $day = $mmdd % 100 - 1;
      return $year * (12 * 31) + $month * 31 + $day;
  }