For VAT purposes, the time of supply is determined as follows:
1. Goods supplied on the invoice basis - The time of supply is the earlier of when the invoice is issued or the payment is received.
a) Goods supplied on the same day as the invoice (1 March) - The time of supply is 1 March, as the invoice was issued on the same day.
b) Goods supplied on 14 May - The time of supply is 14 May, as the invoice was issued on that date.
Therefore, the time of supply for VAT purposes is 1 March for the goods supplied on the same day as the invoice, and 14 May for the goods supplied on 14 May.