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>