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http://www.csgnetwork.com/timezoneproginfo.html

function checkTimeZone() {
 var rightNow = new Date();
 var date1 = new Date(rightNow.getFullYear(), 0, 1, 0, 0, 0, 0);
 var date2 = new Date(rightNow.getFullYear(), 6, 1, 0, 0, 0, 0);
 var temp = date1.toGMTString();
 var date3 = new Date(temp.substring(0, temp.lastIndexOf(" ")-1));
 var temp = date2.toGMTString();
 var date4 = new Date(temp.substring(0, temp.lastIndexOf(" ")-1));
 var hoursDiffStdTime = (date1 - date3) / (1000 * 60 * 60);
 var hoursDiffDaylightTime = (date2 - date4) / (1000 * 60 * 60);
 if (hoursDiffDaylightTime == hoursDiffStdTime) {
      alert("Time zone is GMT " + hoursDiffStdTime +
             ".\nDaylight Saving Time is NOT observed here.");
 } else {
      alert("Time zone is GMT " + hoursDiffStdTime +
             ".\nDaylight Saving Time is observed here.");
   }
}