The date when a transaction takes place for VAT is called its tax point. It is not always the date the supply is actually made. It is vital that you apply the correct tax point to a transaction to ensure that the VAT is declared on the right VAT return and at the correct rate.
The basic tax point is the date at which the goods are made available to the client (or for services the date they are performed). If a VAT invoice is issued before this date, the date of the invoice becomes the tax point. Similarly, if payment is received before the goods are supplied/ work is done, then the date of payment becomes the tax point.
The date a VAT invoice is issued will also become the tax point if it is issued between 1 to 14 days after the supply takes place. If an invoice is issued 15 days or more after, then the date of supply remains the tax point.
It is possible to raise a pro-forma invoice for goods which you have not yet supplied or received payment for. A pro-forma invoice is not a VAT invoice, but it does enable the business to raise a sales document. No VAT is due on this document and it must be clearly display a message such as 'This is not a VAT invoice'.