Services/Form/classes/class.ilDateDurationInputGUI.php and Services/Form/classes/class.ilDateTimeInputGUI.php in ILIAS 5.1.x through 5.3.x before 5.3.4 allow XSS via an invalid date.
The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.