IRS Extending the Tax Assessment Period 

Notice of Deficiency 

If the Commissioner (IRS) determines there is a deficiency of income, estate, gift, or certain miscellaneous excise taxes, the law authorizes the Commissioner to send notice of such deficiency to the taxpayer by certified or registered mail. Certain Service officials are authorized to issue these notices for the Commissioner of Internal Revenue. The notice is not an assessment of tax. It is a proposed deficiency and gives you 90 days (150 days if the notice was addressed to a person outside the United States) to either agree to the deficiency or file a petition with the United States Tax Court for a redetermination of the deficiency. Once the notice of deficiency is issued, the 90 or 150-day period cannot be suspended or extended. The notice of deficiency can be rescinded under certain circumstances if both parties agree. 

IRS Publication 1035 (TTT 05/18/2021)