Hi,
Yes. You can do that. However you will need to use our client side JavaScript API to do because our client side objects are not simple DOM objects. It will be something like this:
eo_GetObject('<%=datePickerBirthdate.ClientID %>').setSelectedDate(new Date(1957, 5, 13));
You can find more information on how to use our client side API here:
http://doc.essentialobjects.com/library/1/clientapi_howto.aspxThanks!