Time of supply of goods: residual rule fixes supply date by return filing deadline or when tax is paid. Section 12(5) provides a residual rule fixing the time of supply of goods, where other subsections do not apply, as the earliest of the date a periodical return must be filed or, otherwise, the date tax is paid; this backstop interacts with invoice timing requirements under Section 31 and governs cases such as advances without invoices and undisclosed supplies or unregistered suppliers where conventional supply dates are indeterminate.