sub find_gaps {
    my $dates = shift;
    my $ndates = scalar keys %$dates;
    my @missing = ();

    return \@missing if $ndates == 0;

    # Determine the first date in the range
    my $year = substr((sort keys %$dates)[0],0,4);
    my $month = 1;
    while ( ! exists $$dates{ "$year-$month" } ) {
        do {$month = 1; ++$year} if ++$month > 12;
    }
    --$ndates;

    # Look for gaps until we've seen each date in the range
    while ( $ndates ) {
        do { $month = 1; ++$year } if ++$month > 12;
        if ( exists $$dates{"$year-$month"} ) {
            --$ndates;
        } else {
            push @missing, "$year-$month";
        }
    }

    return \@missing;
}