The RadDatePicker is a special case because it has two editor control and you could not call just the focus() function. You need to be more specific which element you want to focus.
Please check out the attached web site which demonstrates two approaches, one to show directly the RadDatePicker popup so the user is able to choose directly the date. And in the commented section you could use the code to focus the Dateinput so the user is able to type the date by itself.
You can use the following topic to access the generated picker in edit mode:
( Section Accessing controls in edit/insert mode
I hope this helps. Please give it a try and let me know about the result.
All the best,
the Telerik team
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