 2019  Contract number: [ complete ]  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:  Ir  Pr = Po x ( — )  Io  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.  Payments are deemed to be effected on the date when they are debited to the contracting  authority’s account.  II.21. PAYMENTS AND GUARANTEES  II.21.1  Date of payment  II.21.2  Currency  II.21.3  Conversion  Payments are made in euros or in the currency 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 The costs of the transfer are borne as follows:  II.21.4  Costs of transfer  (a) the contracting authority bears the costs of dispatch charged by its bank;  (b) the contractor bears the costs of receipt charged by its bank;  34 