javascript中的负数if-else条件检查

对于正数，if条件工作正常，但其不执行负数条件可能是错误的原因

//"date is taken as mm/dd/yyyy in input box"
var startDateString = document.getElementById('startDate').value;
var dateToday = new Date();
alert(dateToday);

var myNewDate = process(startDateString);
alert(myNewDate);

var diff = myNewDate - dateToday;
alert(diff);

alert(typeof(diff));

if(diff >= 0){
    alert("selected date is greater than today");
} else {
    alert("selected date is smaller than today");
}

处理功能

function process(date){
    var parts = date.split("/");
    return new Date(parts[2], parts[0] - 1, parts[1]);
}

更改此项：

if(diff >= 0){
  {
   alert("selected date is greater than today");
   } else {
   alert("selected date is smaller than today"); }

例如：

if(diff >= 0){
     alert("selected date is greater than today");
 } else {
   alert("selected date is smaller than today"); 
}

---

为便于日期比较，请转换为历元并进行比较：

   start = startDate.getTime();
   selected = selectedDate.getTime();
   if(selected >= start) {
     // start date was before selected (or dates are equal)
   } else {
     // selected date was before start
   }

---

startDateString里面是什么？diff的值和diff的类型是什么？以2012年11月2日的格式说一个日期。我得到的差异是一个数字。。如果日期越大，则为正数，否则负数看起来好像在if语句后有一个过多的括号。。。