2020 Overpayment applied to 2021 paid with 5/15/21 extension

[dpakman]

According to the new rules, 2020 individual tax returns and payment balances are due 5/15/21, but estimated taxes for 2021 is still due 4/15/21.  If I pay $10,000 with the extension on 5/15/21 and end up with an overpayment of $9,000 to be applied to the 2021 return, is that considered paid on 4/15/21 since overpayments are considered paid for the 1st quarter 2021 estimated taxes or is it considered paid on 5/15/21, the actual date of payment and considered late for the 1st quarter estimated taxes?