jQuery Grid Plugin - jqGrid

As I understand it right now, if I have a custom sorttype like sorttype:sortDate, the function sortDate will only be called when jqGrid is initialized and not the event onSortCol. Is the only way to make onSortcol call SortDate is by manually over riding onSortCol event and writing the tedious code that does this and updates the grid accordingly? Why doesn't defining the sorttype:sortDate just over ride onSortCol event automatically? I mean when the jqGrid gets initailized it sorts correctly, but when I call the event it must do some built in sort. Why I ask is because my sortDate function doesn't have any code that actually updates jqGrid. it just returns 1,-1, or 0. its used in something outside of jqGrid...