You have very useful property - EnableTimeIndependentFiltering for GridDateTimeColumn. But I need to use ItemTemplate and EditItemTemplate and this future missing in GridTemplateColumn. I try to use FilterTemplate with RadDatePicker, but then I have only "equal to" option for filtering. I don't want this. So my suggestion to you is to develop EnableTimeIndependentFiltering for the cases when user have to use GridTemplateColumn with DataType=System.DateTime.
Declined due to lack of demand by the community.