sub new_self_url {
    my %params = @_;
    my $new = new CGI($cgi);
    foreach(keys %params) {
        $new->param($_,$params{$_});
    }
    return $new->self_url;
}

# now you can get urls back to your script with different
# dates like this:
my $newyears = new_self_url(month => 1, day => 1);
my $christmas = new_self_url(month => 12, day => 25);