Hello,
You could use the following client code in order to get the client side object of RadDateInput control.
var input = $find('<%= RadDateInput1.ClientID %>');
.
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The following example shows how to get a reference to its DOM element:
var inputElement = $get('<%=RadInput1.ClientID%>' + '_text');
.
.
Checkout the documentation for more information about this: Client-Side Basics
Additionally, Here is the code that I used for focussing the RadDateInput control from client side. I hope this could help.
-Shinu.