Index: lib/classes/task/scheduled_task.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- lib/classes/task/scheduled_task.php	(date 1421963831000)
+++ lib/classes/task/scheduled_task.php	(revision )
@@ -299,7 +299,12 @@
         // We need to change to the server timezone before using php date() functions.
         $origtz = date_default_timezone_get();
         if (!empty($CFG->timezone) && $CFG->timezone != 99) {
+            if (is_numeric($CFG->timezone)) {
+                $timezone = timezone_name_from_abbr('', ($CFG->timezone * 3600), 0);
+                date_default_timezone_set($timezone);
+            } else {
-            date_default_timezone_set($CFG->timezone);
+                date_default_timezone_set($CFG->timezone);
+            }
         }
 
         $daysinmonth = date("t");
Index: lib/tests/scheduled_task_test.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- lib/tests/scheduled_task_test.php	(date 1421963831000)
+++ lib/tests/scheduled_task_test.php	(revision )
@@ -281,4 +281,26 @@
         $this->assertDebuggingCalled();
         $this->assertNull($task);
     }
+
+    public function test_all_timezones() {
+        global $CFG;
+
+        $this->resetAfterTest(true);
+
+        $timezones = get_list_of_timezones();
+
+        foreach (array_keys($timezones) as $timezone) {
+            $CFG->timezone = $timezone;
+
+            $testclass = new \core\task\scheduled_test_task();
+            $testclass->set_hour('1');
+            $testclass->set_minute('0');
+
+            try {
+                $testclass->get_next_scheduled_time();
+            } catch (Exception $e) {
+                echo "Invalid timezone: $CFG->timezone with error: {$e->getMessage()}\n";
+            }
+        }
+    }
 }