Retroactive Effective Date for 457(b) Plan

Can a 6/30 fiscal year tax-exempt plan sponsor adopt a calendar year nongovernmental 457(b) plan today, having an effective date of 1/1/2022?  The plan will have no employee elective deferrals, so the only contributions will be employer nonelective contributions.  The idea is for the employer to be able to contribute the maximum of $20,500 for 2022 based on full-year compensation of eligible employees by providing for either a retroactive 1/1/2022 entry date or a current entry date while including pre-participation compensation since the beginning of the calendar year.  Relatedly, must employer nonelective contributions for 2022 be made by 12/31/2022?

I think you can do the 2022 calendar year plan year effective 1/1/2022 with employer contributions only. As a nongovernmental 457, this is an unfunded NQDC arrangement. Assets can be set aside or deposited to a rabbi trust, or they can simply be bookkeeping entries. If they are immediately vested, I also think contributions are subject to FICA and Medicare taxes. 

Thanks, CuseFan.  That it's a NQDC plan perhaps raises yet another question about making it retroactively effective.  Is it a permitted deferral, even though it's employer nonelective contribution only, if based on compensation earned before the effective date?  I agree, FICA is payable at vesting.

https://www.irs.gov/retirement-plans/non-governmental-457b-deferred-compensation-plans

The combined employer and employee contribution limit is the lesser of the 402(g) limit or 100% of pay. You obviously could not go retroactive for employee deferrals, but whether you can do that with employer contributions, I'm not sure the more I think about it because they're essentially treated the same except for the necessary timing of elections. So if adopted 12/1 retro to 1/1, would the employer contribution be limited to the lesser of $20,500 or 100% of December's pay? I don't know. I linked IRS website if you might find something helpful in there, I looked quickly but did not find anything specific on this. Maybe someone else on this forum who specializes in these plans can be more helpful.

What the heck do I know? I root for a team that lost to Colgate two years in a row now, yikes!

Plan Doc, per 1.457-4(c)(1)(i)(B) and 1.457-2(g) you're looking at 100% of the individual's 415 comp for the taxable year, so you should be OK for nonelective at least.