Prices are fixed and not subject to revision during the first year of the FWC.  At the beginning of the second and every following year of the FWC, each price may be revised  upwards or downwards at the request of one of the parties.  A party may request a price revision in writing no later than three months before the anniversary date  of entry into force of the FWC. The other party must acknowledge the request within 14 days of  receipt. At the anniversary date, the contracting authority must communicate the final index for the month in  which the request was received, or failing that, the last provisional index available for that month. The  contractor establishes the new price on this basis and communicates it as soon as possible to the  contracting authority for verification. The contracting authority purchases on the basis of the prices in force at the date on which the specific  contract enters into force. The price revision is calculated using the following formula:  where:  Pr = revised price;  Po = price in the tender;  Io = index for the month in which the FWC enters into force;  Ir = index for the month in which the request to revise prices is received.  The date of payment is deemed to be the date on which the contracting authority's account is debited.  Payments are made in euros, unless another currency is provided for in Article I.7. The contracting authority makes any conversion between the euro and another currency at the daily  euro exchange rate published in the Official Journal of the European Union, or failing that, at the  monthly accounting exchange rate, as established by the European Commission and published on the  website indicated below, applicable on the day when it issues the payment order. The contractor makes any conversion between the euro and another currency at the monthly  accounting exchange rate, established by the Commission and published on the website indicated  below, applicable on the date of the invoice.  http://ec.europa.eu/budget/contracts_grants/info_contracts/inforeuro/inforeuro_en.cfm 34  II.21.3 Conversion  II.21.2 Currency  II.21.1 Date of payment  II.21 PAYMENTS AND GUARANTEES  Ir  Pr = Po x (  —  )  Io 