hmm...
In Poll.subs.php, search for:
function modifyPoll($id_poll, $question, $max_votes = 1, $hide_results = 1, $expire = 0, $can_change_vote = 0, $can_guest_vote = 0)
{
$expire = empty($expire) ? 0 : time() + $expire * 3600 * 24;
and remove that last line.
Pay attention the line is present also in createPoll, but that one shall remain!