Timestamp between Javascript and PHP

Javascript:

I have object cell with something date

params.date = cell.getDate();
params.timestamp = cell.getDate().getTime() / 1000;
console.log(params);

Object {date: Thu May 09 2013 00:00:00 GMT+0800 (China Standard Time), timestamp: 1368028800}

Then I try to check timestamp in PHP

$date = '1368028800';
echo date('Y-m-d', $date);

2013-05-08

Difference in one day. Why?