Interest Start Date
If any amount of tax is not paid on or before the last date prescribed for payment, interest on such amount at the underpayment rate established under section 6621 shall be paid for the period from such last date to the date paid. I.R.C. § 6601(a). Similarly, interest shall be paid on any unpaid amount of tax from the last date prescribed for payment of the tax (determined without regard to any extension of time for payment) to the date on which payment is received. Treas. Reg. § 301.6601-1(a)(1). The due date of a gift-tax return is generally April 15th of the year following the year in which the gift was made. I.R.C. § 6075(b). In general, the date prescribed for payment is the time fixed for filing the return, determined without regard to any extension of time for filing. I.R.C. § 6151(a). For determining interest on underpayments, "the last date prescribed for payment" is determined without regard to any extension for payment or filing. I.R.C. § 6601(b)(1). In this case, the assessment will be made for the Year 1 tax year; the due date of the Year 1 gift tax return was Date 1. Underpayment interest will thus run on the assessed deficiency from Date 1. I.R.C. § 6601(a).