倒数计时器javascript

我该如何使倒数计时器对每个人都是相同的，而不管他们在PC上的时间如何。该脚本无法正常运行，基本上任何人都可以通过更改PC上的日期和时间来更改计时器。

var end = new Date('07/31/2015 4:10 pm');

var _second = 1000;
var _minute = _second * 60;
var _hour = _minute * 60;
var _day = _hour * 24;
var timer;

function showRemaining() {
    var now = new Date();
    var distance = end - now;
    if (distance < 0) {

        clearInterval(timer);
        document.getElementById('giveaway1').innerHTML = 'The Winner Has Been Chosen!';

        return;
    }
    var days = Math.floor(distance / _day);
    var hours = Math.floor((distance % _day) / _hour);
    var minutes = Math.floor((distance % _hour) / _minute);
    var seconds = Math.floor((distance % _minute) / _second);

    document.getElementById('giveaway1').innerHTML = days + 'days ';
    document.getElementById('giveaway1').innerHTML += hours + 'hrs ';
    document.getElementById('giveaway1').innerHTML += minutes + 'mins ';
    document.getElementById('giveaway1').innerHTML += seconds + 'secs';
}

timer = setInterval(showRemaining, 1000);