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	(revision bb5135afa27ef56187f545c8700939c456778290)
+++ lib/classes/task/scheduled_task.php	(revision )
@@ -291,7 +291,7 @@
      * @return int $nextruntime.
      */
     public function get_next_scheduled_time() {
-        global $CFG;
+        global $CFG, $DB;
 
         $validminutes = $this->eval_cron_field($this->minute, 0, 59);
         $validhours = $this->eval_cron_field($this->hour, 0, 23);
@@ -299,7 +299,24 @@
         // 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)) {
+                $timezonerow = $DB->get_record('timezone', array('gmtoff' => $CFG->timezone * 60), '*', IGNORE_MULTIPLE);
+
+                // Some of the timezones with a .5 do not map to anything.  For those that do not
+                // map to anything, try rounding down and re-fetching the timezone record.
+                $rounded = round($CFG->timezone, 0, PHP_ROUND_HALF_DOWN);
+                if (empty($timezonerow) && $rounded != $CFG->timezone) {
+                    $timezonerow = $DB->get_record('timezone', array('gmtoff' => $rounded * 60), '*', IGNORE_MULTIPLE);
+                }
+                if (!empty($timezonerow)) {
+                    date_default_timezone_set($timezonerow->name);
+                } else {
+                    debugging("Failed map the site's timezone ($CFG->timezone) to a PHP timezone ID. ".
+                        'Scheduled task next run time might be off from your timezone preference.');
+                }
+            } 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	(revision bb5135afa27ef56187f545c8700939c456778290)
+++ 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";
+            }
+        }
+    }
 }