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.
