It seems that you've upgraded from v.1.x of the Input controls where they had a mask. Since v.2.x of the Classic controls, this is no longer true. The logic of the DateInput component is completely revised and it is a free-form date entry tool now (the masking functionality has been removed). You can find the changes and backward compatibility guide for RadDatePicker here
and for RadInput here
posting here two important help articles as well as a blog post about
the new Input controls, which recommend you for review:
and the blog entry:
Hope these help.
the Telerik team