timezone.js

// run on <select>
$.fn.timeZone = function() {
  return this.extend({
    selectUserTimezone: function(){
      var timeZone = (new Date().getTimezoneOffset() / 60) + ':00'; // => '5:00'
      this.find("option:contains(" + timeZone + "):first").attr('selected', 'selected') 
    }
  })
};

// $('select.time_zone').timeZone().selectUserTimeZone()