**You won’t owe interest or late-payment penalties for the days your check spends in the mail if it is properly mailed and postmarked by the payment deadline.** Under the federal “timely mailing” rule, the USPS postmark date counts as the payment date, provided the IRS receives the payment and your check clears. [Federal mailing rule](https://www.govinfo.gov/content/pkg/USCODE-2023-title26/html/USCODE-2023-title26-subtitleF-chap77-sec7502.htm), [IRS regulations](https://www.irs.gov/pub/irs-regs/td8932.pdf).

Assuming April 15 is your deadline, a check postmarked April 15 and delivered April 18 counts as timely. Your envelope must have the correct IRS address and sufficient postage, and you must deposit it in the U.S. mail by the deadline. **Dating the check April 15 alone doesn’t establish timely payment.** [Mailing requirements](https://www.govinfo.gov/content/pkg/USCODE-2023-title26/html/USCODE-2023-title26-subtitleF-chap77-sec7502.htm).

If mailing on the last day, use USPS certified mail at the counter and keep the postmarked receipt as proof. Follow the IRS’s [check-payment instructions](https://www.irs.gov/payments/pay-by-check-or-money-order) for the address and payment voucher.

Payment is generally due April 15, with adjustments for weekends, holidays, or applicable relief. Filing an extension does **not** extend the payment deadline. [IRS deadlines](https://www.irs.gov/filing/individuals/when-to-file).