TT says 2/18/2021. the actual 1st quarter estimate is not due until 4/15/2021. you have the option of using IRS Direct Pay and avoid having to mail in the voucher and check. you have some leeway in selecting the payment date and at the end you can print a confirmation copy for the payment. you can even cancel the payment if you do it by the date given to you.
https://www.irs.gov/payments
