MDL-26416 avoid subtracting two unsigned ints, which breaks MySQL since 5.5.5.

mod/quiz/report/attemptsreport.php

@@ -149,7 +149,7 @@ abstract class quiz_attempt_report extends quiz_default_report {
             $fields .= "\n(CASE WHEN $qmsubselect THEN 1 ELSE 0 END) AS gradedattempt,";
         }
 
-        $fields .= "
+        $fields .= '
                 quiza.uniqueid AS usageid,
                 quiza.id AS attempt,
                 u.id AS userid,
@@ -164,7 +164,15 @@ abstract class quiz_attempt_report extends quiz_default_report {
                 quiza.sumgrades,
                 quiza.timefinish,
                 quiza.timestart,
-                quiza.timefinish - quiza.timestart AS duration";
+                CASE WHEN qa.timefinish = 0 THEN null
+                         WHEN qa.timefinish > qa.timestart THEN qa.timefinish - qa.timestart
+                         ELSE 0 END AS duration';
+            // To explain that last bit, in MySQL, qa.timestart and qa.timefinish
+            // are unsigned. Since MySQL 5.5.5, when they introduced strict mode,
+            // subtracting a larger unsigned int from a smaller one gave an error.
+            // Therefore, we avoid doing that. timefinish can be non-zero and less
+            // than timestart when you have two load-balanced servers with very
+            // badly synchronised clocks, and a student does a really quick attempt.';
 
         // This part is the same for all cases - join users and quiz_attempts tables
         $from = "\n{user} u";