For about 15 months, Ian has been negotiating a settlement with the IRS concerning a disputed tax

Question:

For about 15 months, Ian has been negotiating a settlement with the IRS concerning a disputed tax deduction. The IRS and Ian have agreed to the amount of prior-year taxes he must pay. Ian asks you whether he also is liable now for interest charges on his underpayment and, if so, how that interest amount is computed. As to Ian's tax payable amount, when does interest begin to accrue?