Must Council Directive 2006/112/EC be interpreted as meaning that tax becomes chargeable on payments on account received by a taxable person supplying tourist services, which are taxed under the special scheme for travel agents provided for in Articles 306 to 310 of Directive 2006/112/EC, at the time defined in Article 65 of Directive 2006/112/EC? If the answer to the first question is in the affirmative, must Article 65 of Directive 2006/112/EC be interpreted as meaning that, for taxation purposes, a payment on account received by a taxable person supplying tourist services, taxed under the special scheme for travel agents provided for in Articles 306 to 310 of Directive 2006/112/EC, is reduced by the cost referred to in Article 308 of Directive 2006/112/EC actually incurred by the taxable person up to the time when he receives the payment on account?