REVENUE REGULATIONS NO. 21-2018 issued on September 14, 2018 provides the regulations to implement Section 249 (Interest) of the National Internal Revenue Code (NIRC) of 1997, as amended under Section 75 of Republic Act No. 10963 (Tax Reform for Acceleration and Inclusion [TRAIN] Law). There shall be assessed and collected on any unpaid amount of tax, interest at the rate of double the effective legal interest rate for loans or forbearance of any money in the absence of an express stipulation as set by the Bangko Sentral ng Pilipinas (BSP). The rate of interest per BSP Memorandum No. 799 series of 2013 for loans or forbearance of any money in the absence of an expressed stipulation is six percent (6%). Thus, the rate of legal interest imposable under Section 249 of the Tax Code, as amended, shall be twelve percent (12%). A Circular shall be issued by the Commissioner in case BSP prescribes new rate of interest. Interest imposed on any deficiency tax due, which interest shall be assessed and collected from the date prescribed for its payment until: a) full payment thereof, or b) upon issuance of a notice and demand by the Commissioner or his authorized representative, whichever comes first. Interest shall be imposed on the failure to pay: a. The amount of the tax due on any return to be filed; or b. The amount of the tax due for which no return is required; or c. A deficiency tax, or any surcharge or interest thereon on the due date appearing in the notice and demand of the Commissioner or his authorized representative until the amount is fully paid, which interest shall form part of the tax. Upon the effectivity of the TRAIN Law (January 1, 2018), in no case shall the deficiency and delinquency interest be imposed simultaneously. In cases where the tax liability/ies or deficiency tax/es became due before the effectivity of the TRAIN Law on January 1, 2018, and where the full payment thereof will only be accomplished after the said effectivity date, the interest rates shall be applied as follows: Period Applicable Interest Type and Rate For the period up to December 31, 2017 Deficiency and/or delinquency interest at 20% For the period January 1, 2018 until full payment of the tax liability Deficiency and/or delinquency interest at 12% The double imposition of both deficiency and delinquency interest under Section 249 prior to its amendment will still apply in so far as the period between the date prescribed for payment until December 31, 2017.