Vaadin 7.3.3, CSS of DateField doesn't gets updated immediately

I’m trying to solve the following problem:

and a
belonging to the same fieldgroup. If I enter
data into the the TextField, Vaadin adds a “v-modified” CSS flag to the control. But if I do the same with the DateField, nothing happens.

The reason why I’m trying to use this flag, is because I want to distinguish between validation errors which are appearing after the GUI has loaded, and a “real” error after the user has entered some data.

I know I could simply set a default value, but this is not an option at the moment.