jQuery Grid Plugin - jqGrid

how to validate two fields in grid form ,eg.begin_date and end_date,begin_date must less than end_date?

colMode:

{ name: 'Begin_Date', label: 'Begin_Date', width: 80, formatter: 'date', formatoptions: { newformat: 'Y-m-d' }, align: "center", editable: true, editoptions: { size: 10, dataInit: function(el) { $(el).datepicker({ dateFormat: 'yy-mm-dd' }); },
                     defaultValue: GetCurrentDate(document.all._Today.value)
                 }, formoptions: { elmsuffix: '*' }, editrules: { required: true, custom: true, custom_func: function(value, colname) { ..... } }
                 },
                 { name: 'End_Date', label: 'End_Date', width: 80, formatter: 'date', formatoptions: { newformat: 'Y-m-d' }, align: "center", editable: true, editoptions: { size: 10, dataInit: function(el) { $(el).datepicker({ dateFormat: 'yy-mm-dd' }); },
                     defaultValue: GetCurrentDate(document.all._Today.value)
                 }, formoptions: { elmsuffix: '*' },editrules: { required: true, custom: true, custom_func: function(value, colname) { ..... }
                 }

function(value, colname) { ..... }??

thanks!