The DatePicker UI widget does not prevent posting an incorrect date. The min/max range restricts the calendar and end-user cannot select a date, which is out of range. It is allowed to type an invalid date, but the DatePicker's value will be null. You can observe this behavior on this online demo.
In order to prevent posting an incorrect value you should use client-side validation.
As a side note, there was a bug related with the "today" link in the calendar. In the demo you can select today's date, even it is not allowed. This issue is already addressed and the fix will be available in the next release of KendoUI.
the Telerik team
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