class ActiveSupport::Duration::ISO8601Serializer

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