When the customs authorities of a Member State authorize a declarant to furnish or supply at a later date certain details concerning the declaration for free circulation of the goods in the form of a periodic declaration, this authorization may, at the declarant's request, provide that a single rate be used for conversion into that Member State's currency of elements forming part of the customs value as expressed in a particular currency. In this case, the rate to be used shall be the rate, established in accordance with this Chapter, which is applicable on the first day of the period covered by the declaration in question.