Must Articles 6 and 16 of Directive 2003/88/EC of 4 November 2003 concerning certain aspects of the organisation of working time be interpreted as imposing a reference period determined on a rolling basis or as allowing Member States to choose whether to employ a rolling or fixed reference period?

If those provisions are to be interpreted as requiring a rolling reference period, may the possibility afforded by Article 17 to derogate from Article 16(b) relate not only to the duration of the reference period but also to the requirement for a rolling period?