Time of Supply of Services: residual rule fixes time as earliest of return filing date or tax payment date. Section 13(5) provides a residual rule for time of supply of services when subsections (2)-(4) do not apply: the time of supply is the earlier of the date on which a periodic return is to be filed or, in other cases, the date on which tax is paid. Section 31's invoice timing provisions (invoice issuance before/within prescribed periods, special rules for continuous supplies, and issuance on cessation) affect which test under section 13 applies, and examples show application where excess receipts are treated as advances or where unregistered suppliers lack documentary dates.