Calculating Time Between Two Dates using Advanced Grid

<script>
    $(function() {
        var grid = FormUtil.getField("advgrid"); // Field id of advance grid
 
        $(grid).on("change", function() {
            var start = FormUtil.getGridCellValues("advgrid.start");
            var end = FormUtil.getGridCellValues("advgrid.end");
 
            for (var i = 0; i < start.length; i++) {
 
                var startDate = new Date(start[i]);
                var endDate = new Date(end[i]);
                var diffMs = (endDate - startDate);
 
                var diffHrs = Math.floor(diffMs / 3600000); // Hours
                var diffMins = Math.round((diffMs % 3600000) / 60000); // Minutes
                var totalDuration = diffHrs + ":" + diffMins;
 
                setValue(grid, i, 3, totalDuration);
 
            }
        });
 
        function setValue(grid, rowIndx, colIndx, colValue) {
            var DM = $(grid).find(".pq_grid").pqGrid("option", "dataModel");
            var data = DM.data;
            var json = data[rowIndx][data[rowIndx].length - 1];
            var obj = eval("[" + json + "]");
 
            var colKey = DM.columnKey[colIndx];
 
            obj[0][colKey] = colValue;
 
            //Update data
            json = JSON.encode(obj);
            json = json.substring(1, json.length - 1);
            DM.data[rowIndx][colIndx + 1] = colValue;
            DM.data[rowIndx][data[rowIndx].length - 1] = json;
 
            //Update hidden table
            var indexKey = DM.data[rowIndx][0];
            var tbl = $(grid).find('.table_container table');
            tbl.find("tr.key_" + indexKey).find("[column_key=" + colKey + "]").text(colValue);
            tbl.find("tr.key_" + indexKey).find("textarea").val(json);
 
            //Update table cell
            var colCell = $(grid).find("tr[pq-row-indx=" + rowIndx + "] .pq-grid-cell[pq-col-indx=" + (colIndx + 1) + "]");
            $(colCell).find(".pq-td-div").html('<div class="form-cell-value"><span>' + colValue + '</span></div>');
        }
    });
</script>