A donor made his first taxable gift in 1979 and his second taxable gift in the current

Question:

A donor made his first taxable gift in 1979 and his second taxable gift in the current year. In the intervening years, the highest gift tax rates declined. In calculating the tax on taxable gifts of previous periods, which rate schedule is applicable: the one for the year in which the donor made the earlier gift or the one for the current period?