sub find_gaps {
  my $dates = shift;
  my @dates = sort{ $a->[0] <=> $b->[0] || $a->[1] <=> $b->[1] }
              map{[substr($_, 0, 4), substr($_, 5)]} keys %$dates;
  my( $minY,$minM,$maxY,$maxM ) = (@{$dates[0]}, @{$dates[-1]});
  my @gaps;
  for my $y ($minY .. $maxY) {
      exists $dates->{"$y-$_"} ? next : push @gaps, "$y-$_"
        for ($minY == $y ? $minM : 1) .. ($maxY == $y ? $maxM : 12);
  }
  return \@gaps;
}