class ActiveSupport::Duration

def sum(sign, time = ::Time.current)
  unless time.acts_like?(:time) || time.acts_like?(:date)
    raise ::ArgumentError, "expected a time or date, got #{time.inspect}"
  end
  if @parts.empty?
    time.since(sign * value)
  else
    @parts.inject(time) do |t, (type, number)|
      if type == :seconds
        t.since(sign * number)
      elsif type == :minutes
        t.since(sign * number * 60)
      elsif type == :hours
        t.since(sign * number * 3600)
      else
        t.advance(type => sign * number)
      end
    end
  end
end