You can use Date.UTC method to get the time stamp at the UTC timezone.
Usage:
var now = new Date;
var utc_timestamp = Date.UTC(now.getUTCFullYear(),now.getUTCMonth(), now.getUTCDate() ,
now.getUTCHours(), now.getUTCMinutes(), now.getUTCSeconds(), now.getUTCMilliseconds());
Live demo here http://jsfiddle.net/naryad/uU7FH/1/