For SMF 1.1.x
The code is in the function ClassifiedsShowListingsRow()
And is controlled by these lines
if ($modSettings['class_catlist_listingend'])
echo '<td>', ($row['expiredate'] == 0 ? $txt['class_expire_never'] : date($modSettings['class_set_date_format_mdy'],$row['expiredate']) . '<br />' . date($modSettings['class_set_date_format_hia'],$row['expiredate']) ) , '</td>';
if ($modSettings['class_catlist_timeleft'])
echo '<td>', ($row['expiredate'] == 0 ? $txt['class_expire_never'] : smf_cattimeleft( date("Y",$row['expiredate']), date("m",$row['expiredate']), date("d",$row['expiredate']), date("H",$row['expiredate']), date("i",$row['expiredate']),date("s",$row['expiredate'])) ) , '</td>';