Timing for Making Company Contributions

The situation is this: Calendar year plan is sponsored by calendar year company A.  An affiliated service group member, company B, has a fiscal year end June 30, with tax return due Sept 15.  My understanding is that company B may make a contribution to the plan for, say, plan calendar year 2020 as late as Sept 15, 2021 and deduct it because part of the plan year (July 1-Dec 31, 2020) fell within company B's fiscal tax year (July 1, 2020-June 30, 2021).  Is this your understanding? Is there a specific IRS ruling to this effect? (Obviously, company A's contribution for plan calendar year 2020 would have been due by Mar 15, 2021.)

I believe the section you are looking for is 1.415(c)-1(b)(6)(B), which says that in order for a contribution to count as an annual addition, it has to be made "no later than 30 days after the end of the period described in section 404(a)(6) applicable to the taxable year with or within which the particular limitation year ends."

Assuming that limitation year = plan year = calendar year, then 2020 limitation year ends on 12/31/2020, which is within B's 7/1/2020-6/30/2021 tax year, and the 404(a)(6) period for that tax year would end on 9/15/2021, extensions notwithstanding.

You may also find 1.404(a)-14(c) relevant, which talks about how to determine the deductible limit when the tax year is different than the plan year.