Fix course date change not reflected in due dates due to caching

classes/studyplanservice.php

@@ -1235,7 +1235,9 @@ class studyplanservice extends \external_api
                 'reset_start_date_old' => $course->startdate,
                 'reset_end_date_old' => $course->enddate,
             ]);
-
+            // purge course cache so the dates are properly reflected
+            \course_modinfo::purge_course_cache($course->id);
+            
             return (new courseinfo($course->id))->editor_model();
         } else {
             // probably should return a nice message