expires_at.rb

# expires_at '21:30', :public => true, :private => false
# will set max-age to the amount of seconds between now and tomorrow at 9:30PM

module ExpiresAt
  
  def self.included(base)
    base.send(:include, InstanceMethods)
  end
  
  module InstanceMethods
   
    def expires_at_midnight(options={})
      now = Time.now
      cache_control = response.headers["Cache-Control"].split(",").map {|k| k.strip }
      cache_control << "max-age=#{(now.end_of_day - Time.now).to_i}"
      cache_control.delete("no-cache")
      if options[:public]
        cache_control.delete("private")
        cache_control << "public"
      else
        cache_control << "private"
      end

      cache_control += options.symbolize_keys.reject{|k,v| k == :public || k == :private }.map{ |k,v| v == true ? k.to_s : "#{k.to_s}=#{v.to_s}"}

      response.headers["Cache-Control"] = cache_control.join(', ')
    end
    end

    def expires_at(time='00:00', options = {})
      h, m = time.to_s.split(':')
      h = h.to_i
      m = m.to_i
      h = 0 unless h.between?(0, 23)  
      m = 0 unless m.between?(0, 59)
      seconds = ((Time.now.end_of_day + h.hours + m.minutes) - Time.now).to_i
      cache_control = response.headers["Cache-Control"].split(",").map {|k| k.strip }
      cache_control << "max-age=#{seconds}"
      cache_control.delete("no-cache")
      if options[:public]
        cache_control.delete("private")
        cache_control << "public"
      else
        cache_control << "private"
      end

      cache_control += options.symbolize_keys.reject{|k,v| k == :public || k == :private }.map{ |k,v| v == true ? k.to_s : "#{k.to_s}=#{v.to_s}"}

      response.headers["Cache-Control"] = cache_control.join(', ')
    end
  end
end

ActionController::Base.send(:include, ExpiresAt)