Ir  Pr = Po x (  —  )  Io  where:  Pr = revised price;  Po = price in the tender;  Io = index for the month in which the contract 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 The costs of the transfer are borne as follows:  If, as provided for in Article I.6 ,  a financial guarantee is required for the payment of pre-financing, as  performance guarantee or as retention money guarantee, it must fulfil the following conditions: Pre-financing guarantees must remain in force until the pre-financing is cleared against interim  payments or payment of the balance. Where the payment of the balance takes the form of a debit  note, the pre-financing guarantee must remain in force for three months after the debit note is sent  to the contractor. The contracting authority must release the guarantee within the following month. The contractor bears the cost of providing such guarantee.  31  (b) the guarantee shall have the effect of making the bank or financial institution or the third party  provide irrevocable collateral security, or stand as first-call guarantor of the contractor's  obligations without requiring that the contracting authority has recourse against the principal  debtor (the contractor).  (a) the financial guarantee is provided by a bank or a financial institution approved by the  contracting authority or, at the request of the contractor and with the agreement of the  contracting authority, by a third party; and  (c) the party causing repetition of the transfer bears the costs for repeated transfer.  (b) the contractor bears the costs of receipt charged by its bank;  (a) the contracting authority bears the costs of dispatch charged by its bank;  II.21 PAYMENTS AND GUARANTEES  II.21.1. Date of payment  II.21.2. Currency  II.21.3. Conversion  II.21.4. Costs of transfer  II.21.5. Pre-financing, performance and money retention guarantees 