http://plnkr.co/edit/oakPie?p=preview
Your brushWidth was a string. Add the "+" in front to change it to an int.
chartXScale = d3.time.scale().range([0, +brushWidth]).domain([
new Date("Tue Feb 11 2014 14:37:00 GMT-0500 (EST)"),
new Date("Tue Feb 11 2014 16:37:00 GMT-0500 (EST)")
]);