When using set_col_date() method, it's important to correctly set the 3rd parameter. The 3rd parameter is used by the datepicker which must conform to the same datetime format used in the database for that column (e.g. orderDate), or the value won't save properly. See: http://phpgrid.com/documentation/set_col_date/
 
$dg -> set_col_date("orderDate", "Y-m-d", "d.m.Y", "yy-mm-dd");

In addition to set_col_date() method, you can also use set_col_property(). To display time only, you must use set_col_property() method. See: http://phpgrid.com/documentation/set_col_property/