JavaScript setTimeout如此不准确的原因是什么？

I got this code over here:

我在这里得到了这个代码:

var date = new Date();
setTimeout(function(e) {
    var currentDate = new Date();
    if(currentDate - date >= 1000) {
         console.log(currentDate, date);
         console.log(currentDate-date);
    }
    else {
       console.log("It was less than a second!");
       console.log(currentDate-date);
    }
}, 1000);
var