Article 18 of the KSA Value Added Tax Law establishes the definitive rule for determining the applicable tax rate. It stipulates that the VAT rate due on a transaction is the rate that is legally in force on the specific date the supply of goods or services occurs. For goods being brought into the Kingdom from abroad, the determining factor is the official date of import. This provision provides legal certainty for taxpayers, ensuring that any subsequent amendments to the tax rate do not retroactively affect transactions that have already taken place based on their established supply or import date.