Javascript计时器在后台

Question

我有一个工作计时计时器，但每次更改页面时都会重置，例如index.php -> help.php -> index.php（这里计时器从0开始，而不是在后台计数）

代码在这里：

<h1 id="demo"></h1>

countDownDate = new Date(); localStorage.setItem('startDate', countDownDate);
var countDownDate = localStorage.getItem('startDate');
if (countDownDate) {
    countDownDate = new Date(countDownDate);
} else {
    countDownDate = new Date();
    localStorage.setItem('startDate', countDownDate);
}
// Update the count down every 1 second
var x = setInterval(function() {

    // Get todays date and time
    var now = new Date().getTime();

    // Find the distance between now an the count down date
    var distance = now - countDownDate.getTime();

    // Time calculations for days, hours, minutes and seconds
    var days = Math.floor(distance / (1000 * 60 * 60 * 24));
    var hours = Math.floor((distance % (1000 * 60 * 60 * 24)) / (1000 * 60 * 60));
    var minutes = Math.floor((distance % (1000 * 60 * 60)) / (1000 * 60));
    var seconds = Math.floor((distance % (1000 * 60)) / 1000);

    // Output the result in an element with id="demo"
    document.getElementById("demo").innerHTML = days + "d " + hours + "h " + minutes + "m " + seconds + "s ";
}, 1000);