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@jskouberdis wrote:

I cannot find anything definite on the internet. 

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After discussing the amount of the penalty per month, IRM 20.1.2.6.2 says:

"For the purpose of computing this penalty, the definition of "month" is the same as it is for computing the penalty for filing late under IRC 6651(a)(1). See IRM 20.1.2.3.7.1, Period Subject to IRC 6651(a)(1)."

Then when you look up IRM 20.1.2.3.7.1, it says (bold/underlines is mine):

"The period subject to the penalty for filing late under IRC 6651(a)(1) begins on the day following the latest of the following dates:

https://www.irs.gov/irm/part20/irm_20-001-002r#idm139677102681616

https://www.irs.gov/irm/part20/irm_20-001-002r#idm139677103507904