application.js

jQuery(function() {
  $.cookie('tz', (new Date()).getTimezoneOffset());
});

application_controller.rb

class ApplicationController < ActionController::Base
  before_filter :set_timezone
  
  # …
private

  def set_timezone
    if current_user && browser_timezone && browser_timezone.name != current_user.time_zone
      current_user.update_attributes :time_zone => browser_timezone.name
    end
    Time.zone = current_user ? current_user.time_zone : browser_timezone
  end
  
  def browser_timezone
    @browser_timezone ||= begin
      ActiveSupport::TimeZone[-cookies[:tz].to_i.minutes]
    end if cookies[:tz].present?
  end

end