Article 25 establishes the definitive 'date of supply' for the purpose of calculating tax on goods and services, stipulating that the tax obligation arises on the earliest of several specified dates. For goods, this includes the date of transfer (whether supervised or not), completion of assembly, or the import date under Customs Legislation. For returnable supplies, the date is tied to the recipient's acceptance or a maximum of 12 months post-transfer. For services, the trigger is the date of completion. Critically, the date of receiving payment or issuing the Tax Invoice also establishes the supply date if it precedes other events.