To reproduce: please refer to the attached sample project and gif file. 

Workaround:

private void radGridView1_CellEditorInitialized(object sender, GridViewCellEventArgs e)
        {
            RadDateTimeEditor editor = e.ActiveEditor as RadDateTimeEditor;
            if (editor != null)
            {
                RadDateTimeEditorElement el = editor.EditorElement as RadDateTimeEditorElement;
                if (el != null)
                {
                    el.TextBoxElement.TextBoxItem.GotFocus -= TextBoxItem_GotFocus; 
                    el.TextBoxElement.TextBoxItem.GotFocus += TextBoxItem_GotFocus; 
                }
            }
        }

        private void TextBoxItem_GotFocus(object sender, EventArgs e)
        {
            RadTextBoxItem tb = sender as RadTextBoxItem;
            if (tb != null)
            {
                tb.SelectionLength = 0;
            }
        }