(1) which is contributed by an employer to a trust described in (relating to withdrawal liability payment fund) which meets the requirements of of the Employee Retirement Income Security Act of 1974, and
(2) which is properly allocable to such taxable year.
In the case of a contribution described in subsection (a) which relates to any specified period of time which includes more than one taxable year, the amount properly allocable to any taxable year in such period shall be determined by prorating such amounts to such taxable years under regulations prescribed by the Secretary.
No deduction shall be allowed under subsection (a) with respect to any contribution described in subsection (a) which does not relate to any specified period of time.