IRS Extending the Tax Assessment Period 

Interest 

Generally, interest continues to accrue on any balance due until full payment is made, including the time for appeal within the Service or the courts. However, for individual taxpayers, interest on an income tax deficiency will stop accruing under certain conditions after 18 months (36 months if the Service notifies you of the additional tax liability after November 25, 2007) from the later of the return due date or the date of filing if filed under a valid filing extension and will start accruing again if payment is not timely made after the Service notifies you of the additional tax liability. Also, a cash deposit may be made to stop the accrual of interest. For information on cash deposits, please contact the Service employee requesting the consent. 

IRS Publication 1035 (TTT 07/20/21 and 07/27/21)