What is the last date on which a payment by way of advance tax is treated as advance tax for that financial year under section 408(3)?

Section 408(3) treats any amount paid by way of advance tax on or before 31 March as advance tax paid during the financial year ending on that day. The last instalment falls due on 15 March, but a payment made between 16 and 31 March still counts as advance tax for that year.