更好的捷径:

+new Date # Milliseconds since Linux epoch
+new Date / 1000 # Seconds since Linux epoch
Math.round(+new Date / 1000) #Seconds without decimals since Linux epoch

要从Javascript epoch中获取秒数，请使用:

date = new Date();
milliseconds = date.getTime();
seconds = milliseconds / 1000;

var seconds = new Date().getTime() / 1000;

．...能告诉你从1970年1月1日午夜开始的秒数吗

参考

 Date.now()

给出从epoch开始的毫秒数。不需要使用new。

在这里查看参考资料:https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/now

(IE8不支持)

根据你的评论，我认为你在寻找这样的东西:

var timeout = new Date().getTime() + 15*60*1000; //add 15 minutes;

然后在你的检查中，你在检查:

if(new Date().getTime() > timeout) {
  alert("Session has expired");
}

这些JavaScript解决方案提供了从1970年1月1日午夜开始的毫秒或秒。

IE 9+解决方案(IE 8或旧版本不支持):

var timestampInMilliseconds = Date.now();
var timestampInSeconds = Date.now() / 1000; // A float value; not an integer.
    timestampInSeconds = Math.floor(Date.now() / 1000); // Floor it to get the seconds.
    timestampInSeconds = Date.now() / 1000 | 0; // Also you can do floor it like this.
    timestampInSeconds = Math.round(Date.now() / 1000); // Round it to get the seconds.

要获得有关Date.now()的更多信息，请访问https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/now

通用解决方案:

// ‘+’ operator makes the operand numeric.
// And ‘new’ operator can be used without the arguments ‘(……)’.
var timestampInMilliseconds = +new Date;
var timestampInSeconds = +new Date / 1000; // A float value; not an intger.
    timestampInSeconds = Math.floor(+new Date / 1000); // Floor it to get the seconds.
    timestampInSeconds = +new Date / 1000 | 0; // Also you can do floor it like this.
    timestampInSeconds = Math.round(+new Date / 1000); // Round it to get the seconds.

如果你不想要这种情况，请小心使用。

if(1000000 < Math.round(1000000.2)) // false.