Measure the elapsed time between sending the request and getting back the response. Then, divide that value by 2. That gives you a rough value of one-way latency. If you add that to the time value from the server, you’ll be closer to the true server time.
Something like this should work:
function syncTime() {
// Set up our time object, synced by the HTTP DATE header
// Fetch the page over JS to get just the headers
console.log("syncing time")
var r = new XMLHttpRequest();
var start = (new Date).getTime();
r.open('HEAD', document.location, false);
r.onreadystatechange = function()
{
if (r.readyState != 4)
{
return;
}
var latency = (new Date).getTime() - start;
var timestring = r.getResponseHeader("DATE");
// Set the time to the **slightly old** date sent from the
// server, then adjust it to a good estimate of what the
// server time is **right now**.
systemtime = new Date(timestring);
systemtime.setMilliseconds(systemtime.getMilliseconds() + (latency / 2))
};
r.send(null);
}
Interesting aside: John Resig has a good article on the accuracy of Javascript timing.
It shouldn’t cause a problem in this case, since you’re only concerned about your time being off by ~1 second. A 15 ms difference shouldn’t have much effect.