When a date is selected you see the date appearing in the readonly textbox.

Now I wanted to use a required validation on the textbox but when there is no date selected I get a correct error message saying I have to choose a date so that's in order but when I then click on the textbox and select a date it doesn't update. It keeps saying that there's an error.But when I do it a second time the error message is cleared. 

Does anyone know how to fix this.

Link: http://oerl-express.be/default.php?page=reserveren (it's in dutch though)