I have already answered your support ticket on the same subject, however I am pasting my reply here so the other users who are interested in the same can read it:
I am afraid what you are trying to achieve is not supported in Kendo UI Date/TimePickers and its implementation requires additional code. As a possible approach you can hook up to the change event of the widgets, calculate the last day of the month and set the new date.
the Telerik team
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