serialize

# File activesupport/lib/active_support/duration/iso8601_serializer.rb, line 15
      def serialize
        parts = normalize
        return "PT0S" if parts.empty?

        output = +"P"
        output << "#{parts[:years]}Y"   if parts.key?(:years)
        output << "#{parts[:months]}M"  if parts.key?(:months)
        output << "#{parts[:days]}D"    if parts.key?(:days)
        output << "#{parts[:weeks]}W"   if parts.key?(:weeks)
        time = +""
        time << "#{parts[:hours]}H"     if parts.key?(:hours)
        time << "#{parts[:minutes]}M"   if parts.key?(:minutes)
        if parts.key?(:seconds)
          time << "#{format_seconds(parts[:seconds])}S"
        end
        output << "T#{time}" unless time.empty?
        output
      end