To reproduce:
Add a grid with a DateTime column and change its editor to GridTimePickerEditor
Run the project and on a row with no value, open the editor, press Up arrow key, change to AM and then change the hour to 12
Open the popup and click the left key to change the AM/PM

Work around:
  void radGridView1_EditorRequired(object sender, Telerik.WinControls.UI.EditorRequiredEventArgs e)
        {
            if (e.EditorType == typeof(RadDateTimeEditor))
            {
                if (radGridView1.CurrentColumn.Name == "TimeColumn")
                {
                    GridTimePickerEditor editor = new GridTimePickerEditor();
                    e.Editor = editor;
                    RadTimePickerElement element = editor.EditorElement as RadTimePickerElement;
                    element.PopupContentElement.HoursTable.TableHeader.LeftArrow.MouseDown -= new MouseEventHandler(LeftArrow_MouseDown);
                    element.PopupContentElement.HoursTable.TableHeader.LeftArrow.MouseDown += new MouseEventHandler(LeftArrow_MouseDown);
                }
            }
        }

        void LeftArrow_MouseDown(object sender, MouseEventArgs e)
        {
            RadTimePickerElement element = ((GridTimePickerEditor)this.radGridView1.ActiveEditor).EditorElement as RadTimePickerElement;
            DateTime time = (DateTime)element.Value;
            element.Value = time.AddYears(-2);
        }