Failure to pay penalty for one week delay after filing timely extension?

[twistedether]

I am filing a timely extension (before April 15th) but sending no check to IRS.  If I complete my tax filing one week after the April 15th deadline and sent the check then, would IRS still charge me failure to pay penalty interest for even one week?  I ask because my amount could be big, i.e. about $200K.  So one week accrual is important. But I don't know for sure if I owe that amount or far less.  That is why an accountant has to double check, who doesn't have time before April 15th.

[linear]

Yes, penalties start to accrue if you don't pay by April 15.

"Failure to pay – when you don't pay the taxes reported on your return in full by the due date, April 15. An extension to file doesn't extend the time to pay." (https://www.irs.gov/businesses/small-businesses-self-employed/understanding-penalties-and-interest)