Thanks for your info.
I have re-tested to reopen the certain closed ticket, afterwards, the duedate is still NULL, and I think it should be! Why? Since I saw from the related code in the class.ticket.php file, around line 387 at the reopen function:
$sql= 'UPDATE '.TICKET_TABLE.' SET status='.db_input('open').',isanswered=0,updated=NOW(),reopened=NOW() WHERE ticket_id='.db_input($this->getId());
From that part of code, we see that there is no any code which will update the duedate field.
I just wondered whether the weird condition you guys mentioned above only happened to the certain MySQL version? :
Anyway, mine is 5.0.51b and everything seemed okay so far.
Sincerely,
Masino Sinaga