The GridTimePickerEditor format is not correctly taken from the GridDateTimeColumn FormatString property. Workaround: Use the Format and CustomFormat properties.
More information how to customize the format of date can be fount in our help on the following link: http://www.telerik.com/help/winforms/gridview-columns-gridviewdatetimecolumn.html
The value of the FormatString property is used only when formatting the value inside the cell for display purposes. For editing, there are several properties that control the format of the value in the editor of a DateTime column: Format - can be Long, Short, Time or Custom CustomFormat - used when the Format property is set to Custom