PAYMENT OF TAX

As per S. 12(2) , time of supply shall be earlier of-
1. Date of issue of invoice or last date to issue invoice u/s 31.
2. Date of receipt of payment.
However , I read that payment when an advance is received tax need not be paid.
Tax on it shall be paid only at the time of issue of invoice.
Isn't it the case of receipt of payment , then why can't the tax be paid at that moment.