Quiz

I have the same problem after OS upgrade with moodle 1.9.9the solution is to change around line 344 in /mod/quiz/report/overview/report.php 

from

$select .= 'qa.uniqueid AS attemptuniqueid, qa.id AS attempt, '  
'u.id AS userid, u.idnumber, u.firstname, u.lastname, u.picture, u.imagealt, ' 
'qa.sumgrades, qa.timefinish, qa.timestart, qa.timefinish - qa.timestart AS duration';

to

$select .= 'qa.uniqueid AS attemptuniqueid, qa.id AS attempt, '
'u.id AS userid, u.idnumber, u.firstname, u.lastname, u.picture, u.imagealt, ' 
'qa.sumgrades, qa.timefinish, qa.timestart, convert(qa.timefinish,signed) - convert(qa.timestart,signed) AS duration';