Zero coupon bond discount computation sets a month-based formula for prorating discount across the tax year. Prescribes the method for computing the pro rata amount of discount on a zero coupon bond for the purpose of section 32(d). The bond's life is converted into calendar months, with parts of a month of fifteen days or more counted as a full month and parts of less than fifteen days ignored. The total discount is divided by the resulting number of months, and the amount attributable to a tax year is determined by multiplying the monthly figure by the months included in that year.