((a)) ** Computation of tax** The tax imposed by section 2501 for each calendar year shall be an amount equal to the excess of—

((1)) a tentative tax, computed under section 2001(c), on the aggregate sum of the taxable gifts for such calendar year and for each of the preceding calendar periods, over

((2)) a tentative tax, computed under such section, on the aggregate sum of the taxable gifts for each of the preceding calendar periods.

((b)) ** Preceding calendar period** Whenever used in this title in connection with the gift tax imposed by this chapter, the term “preceding calendar period” means—

((1)) calendar years 1932 and 1970 and all calendar years intervening between calendar year 1932 and calendar year 1970,

((2)) the first calendar quarter of calendar year 1971 and all calendar quarters intervening between such calendar quarter and the first calendar quarter of calendar year 1982, and

((3)) all calendar years after 1981 and before the calendar year for which the tax is being computed.

((c)) ** Tax to be paid by donor** The tax imposed by section 2501 shall be paid by the donor.