VATis an instant tax and accrues operation by operation, but from the standpoint of management, his statement is regular, usually quarterly, and each statement, the taxpayer includes the accrued sum of all operations, and subtracts the taxes charged in the period to pay the difference.
But being an instant tax, when the accrual occurs? The general rule, that is, for the supply of goods, is accrued in the time it gets to the purchaser the right, and in operations and services, at the time the service is rendered materially. If VAT is payable when the good is delivered, that is the time to be declared.
In the case of advance payments, there is a special rule, which is that VAT is chargeable on each partial advance payment for the invoiced amount. This occurs, for example, when someone buys a house under construction.
There are more special cases, for example in the case of successive chain operations, such as leases, where the accrual occurs at the time when the party becomes due price corresponding to each perception.