求 一js
如何把2个字符串相减得到毫秒数

比如var a='2013-12-29 13:23:33';
var b='2013-12-27 15:33:54';

a-b得到毫秒数，需要能在ie8下也能正常运行的js脚本
[解决办法]

var a = Date.parse("Dec 29, 2013, 13:23:33"),
b = Date.parse("Dec 27, 2013, 15:23:54");

alert(a-b)

var a = "2013-12-29 13:23:43";
var b = "2013-12-29 13:23:33";

var date1 = new Date(Date.parse(a.replace(/\-/g, "/")));
var date2 = new Date(Date.parse(b.replace(/\-/g, "/")));

//得出10毫秒
alert(date1.getTime()/1000-date2.getTime()/1000);