We have already replied to your support ticket. For those members of the community, who might be interested in your case I am posting the answer here, as follows:
You may try to handle PreparingCellForEdit/ PreparedCellForEdit events of RadGridView from where you may access the corresponding Editing Element.
Once you get the RadDateTimePicker instance you can set the date format by changing the Culture property, something like the following:
sender, GridViewPreparingCellForEditEventArgs e)
) e.Column.Header ==
var rdt = e.EditingElement
the Telerik team