class ActiveSupport::Duration

def sum(sign, time = ::Time.current) #:nodoc:
  parts.inject(time) do |t,(type,number)|
    if t.acts_like?(:time) || t.acts_like?(:date)
      if type == :seconds
        t.since(sign * number)
      else
        t.advance(type => sign * number)
      end
    else
      raise ::ArgumentError, "expected a time or date, got #{time.inspect}"
    end
  end
end