Recuperer l'heure locale du visiteur

/* Fonction qui stocke le timestamp local dans $_SESSION['localTime'] */
function sendTimestamp() {
	var date = new Date();
	var timestamp = Math.floor(date.getTime() / 1000);

	var xhr = null;

	if (window.XMLHttpRequest || window.ActiveXObject) {
		if (window.ActiveXObject) {
			try {
				xhr = new ActiveXObject("Msxml2.XMLHTTP");
			} catch(e) {
				xhr = new ActiveXObject("Microsoft.XMLHTTP");
			}
		} else {
			xhr = new XMLHttpRequest(); 
		}
	} else {
		alert("Votre navigateur ne supporte pas l'objet XMLHTTPRequest...");
		return;
	}

	xhr.onreadystatechange = function() {
		if(xhr.readyState == 4 && (xhr.status == 0 || xhr.status == 200)) {
			return true;
		}
	};

	xhr.open('POST','recv_client_timestamp.php',true);
	xhr.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
	xhr.send('timestamp='+timestamp);
}

<?php
session_start();
$_SERVER['localTimestamp']['server'] = $_SERVER['REQUEST_TIME'];
$_SERVER['localTimestamp']['client'] = (int) $_POST['timestamp'];

<?php
session_start();

function getClientTimestamp() {
	$diff = $_SERVER['REQUEST_TIME'] - $_SESSION['localTimestamp']['server'];
	return $_SERVER['localTimestamp']['client'] + $diff;
}