Yea.... I did try that and it did not work for me. I also tried defining an ID for the <DateInput > tag/control and defined runat="server" and didn't work either.
However, using the tip you provided (and "viewing the source" of the page) I was able to get it to work using
I was also able to the the radcombobox working using
RadAjaxManager1.FocusControl(cboMyRadCombo.ClientID & "_input")