Conversion of datetime strings in PHP pages to user's timezone
I would think it would be easier and more reliable to use timezone offsets rather than exact server time vs. client time. You can get the client-side offset using new Date().getTimezoneOffset() which returns the offset in minutes from UTC. When the timezone is +2 then the offset is -120. 

PHP

&lt;meta name="timezoneoffset" content="&lt;?php echo date('P') ?&gt;" /&gt;


HTML

&lt;meta name="timezoneoffset" content="+0200" /&gt;


JavaScript

var serverTimeZone= $("meta[name='timezoneoffset']").attr("content").substring(0,3);
// getTimezoneOffset returns (client timezone * -60). 
// With timezones the the formula would be (clientTz - serverTz) * 60 * 60000
var timezoneOffset = (new Date().getTimezoneOffset() + (serverTimeZone * 60)) * 60000;
$("span.datetime-convert").each(function(){
    var date = parseDatetime($(this).text());
    if(date) { // If date is not wellformed the script would crash without this guard
      var offsetDate = new Date(date.getTime() + timezoneOffset);
      $(this).text(offsetDate .toLocaleString());
    }
});
function parseDatetime(value) {
    var a = /^(\d{4})-(\d{2})-(\d{2}) (\d{2}):(\d{2}):(\d{2})$/.exec(value);
    if (a) {
        return new Date(+a[1], +a[2] - 1, +a[3], +a[4], +a[5], +a[6]);
    }
    return null;
}


Also there's no need to calculate the offset for each individual date. It can be actually a bit misleading when the offset is slighlty different for each date.

http://jsfiddle.net/xXVPC/1/

More functional: 

var serverTimeZone= $("meta[name='timezoneoffset']").attr("content").substring(0,3);
// getTimezoneOffset returns (client timezone * -60). 
// With timezones the the formula would be (clientTz - serverTz) * 60 * 60000
var timezoneOffset = (new Date().getTimezoneOffset() + (serverTimeZone * 60)) * 60000;
$("span.datetime-convert").each(function(){
    var e = $(this);
    parseDatetime(e.text(), new function(date) {
      var offsetDate = new Date(date.getTime() + timezoneOffset);
      e.text(offsetDate.toLocaleString());
    });
});
function parseDatetime(value, callback) {
    var a = /^(\d{4})-(\d{2})-(\d{2}) (\d{2}):(\d{2}):(\d{2})$/.exec(value);
    if (a) {
        callback(new Date(+a[1], +a[2] - 1, +a[3], +a[4], +a[5], +a[6]));
    }
}