add sign_up_start_at

app/Models/Course.php

@@ -18,7 +18,9 @@ class Course extends Model
         'created_at',
         'updated_at',
         'start_at',
-        'end_at'
+        'end_at',
+        'sign_up_start_at',
+        'sign_up_end_at',
     ];
 
 }