What is the football player Francois Affolter header's finishing rate on 18/09/2014?
header's finishing rate refers to heading_accuracy; on 18/09/2014 refers to date = '2014-09-18 00:00:00';
SELECT t2.heading_accuracy FROM Player AS t1 INNER JOIN Player_Attributes AS t2 ON t1.player_api_id = t2.player_api_id WHERE t1.player_name = 'Francois Affolter' AND SUBSTR(t2.`date`, 1, 10) = '2014-09-18'