Add validation rule so session end date must be after session start date

add_form.php

@@ -158,6 +158,10 @@ class mod_attendance_add_form extends moodleform {
     public function validation($data, $files) {
         $errors = parent::validation($data, $files);
 
+        if ($data['sessiondate'] != 0 && $data['sessionenddate'] != 0 && $data['sessionenddate'] < $data['sessiondate']) {
+            $errors['sessionenddate'] = get_string('invalidsessionenddate', 'attendance');
+        }
+
         if ($data['sessiontype'] == attendance::SESSION_GROUP and empty($data['groups'])) {
             $errors['groups'] = get_string('errorgroupsnotselected', 'attendance');
         }

lang/en/attendance.php

@@ -121,6 +121,7 @@ $string['identifyby'] = 'Identify student by';
 $string['includeall'] = 'Select all sessions';
 $string['includenottaken'] = 'Include not taken sessions';
 $string['indetail'] = 'In detail...';
+$string['invalidsessionenddate'] = 'The session end date can not be earlier than the session start date';
 $string['jumpto'] = 'Jump to';
 $string['modulename'] = 'Attendance';
 $string['modulename_help'] = 'The attendance activity module enables a teacher to take attendance during class and students to view their own attendance record.