sum

# File activesupport/lib/active_support/duration.rb, line 157
      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)
            elsif type == :minutes
              t.since(sign * number * 60)
            elsif type == :hours
              t.since(sign * number * 3600)
            else
              t.advance(type => sign * number)
            end
          else
            raise ::ArgumentError, "expected a time or date, got #{time.inspect}"
          end
        end
      end