The tax point of supplies covered by self-billed invoices follows the normal rules, except that a self-billed invoice is only effective to create a tax point when issued within 14 days of the basic tax point.
The time of supply is normally determined by the date of despatch of goods or performance of services (basic tax point), unless this is overridden by the date of payment. In any event, the client must show the tax point on the invoice.