[Issvar]

Open forumdisplay.php, and find

$sort = array();

And add before this line:

if ($forumid == 5) {
  $sortfield = 'dateline';
  $sqlsortorder = 'DESC';
}

And replace the 5 with the id number of the forum you wish to have sorted by start time. Now your threads will always be sorted with newest first. If you would prefer oldest first change DESC to ASC.

You can find the id number of your forum by simply opening that forum and looking in the address bar at what it says after &forumid=.