演示： http://jsfiddle.net/AuhtS/

代码：

var a = new Date();
var b = new Date();
console.log(b - a); //this works
localStorage.a = a;
localStorage.b = b;
a = Date.parse(localStorage.a); // parse to date object
b = Date.parse(localStorage.b);
console.log(b - a); // now, this will work

原因

所有内容都存储为 localStorage 中的字符串。

Everything is stored as a string in localStorage.

因此，当您执行 localStorage.b - localStorage.a 时，您尝试的是尝试从另一个字符串中减去一个字符串。这就是为什么它不起作用。

So when you do localStorage.b - localStorage.a, what you're attempting is trying to subtract one string from another. Which is why it doesn't work.