Employer contribution timing to a nonprofit 457(b)

What is the deadline for a nonprofit organization to make an employer match or nonelective contribution to a 457(b) plan for a plan year?  

While (a) your question might be zeroing in on the answer:  Whenever the plan documents say contributions must be delivered; 

(b) your question suggests a misunderstanding about unfunded and funded 457(b) plans for nonprofits and the tax consequences.

Apologies if I have misapprehended.

Let's assume the contributions if any are completely discretionary.  The key is that the the payroll taxes are due "at the time of the deferral"

https://www.irs.gov/pub/irs-drop/n-03-20.pdf

“Annual deferrals,” as used in this notice, means the amount of compensation deferred under the plan in accordance with § 457(b), and in compliance 4 with the annual maximum deferral limitation under the plan, whether by elective deferral or nonelective employer contribution, during a taxable year.  [whatever this last part means...]

When is the EMployer Contribution deferred?  Let's assume the contribution is discretionary.  If it is discretionary, then unless/until the money is set aside in a separate account (within a Rabbi trust for example), I would say it is reasonable to assume it has not yet been deferred.

On the other hand, if the Board declares in October 2018 that the Employer will contribute $15,000 for the CEO, and then sets that money aside in March of 2019, presumably it would be taxable for FICA in 2018.   I'm assuming no vesting schedule.

Of course for the payroll deduction stuff it is easy.  The day it is withheld it is deferred.

Ok, I think I understand.  So, as long as an employer says they set aside an amount for a tax year, it doesn't really matter that it was physically deposited into an account during that year.  That would mean there is no deadline agree?  They can say $15,000 was set aside October 2018 but even if never deposited into an account, it doesn't matter.  What matters is when the amount was FICa taxed...

The plan document matters.  It is a contract.  If the contract says that some amount will be put in some account by some date, then it is a breach of contract not to do so.  This would most likely be an issue if the participant directs the investment of an account, as a practical matter, and not as a legal right in an unfunded (for tax purposes) plan.  If the accrual is only a bookkeeping accrual, with a bookkeeping investment rate, then the plan usually will not speak about contributions.  Instead, it will speak about accrual credits.  The timing of a legal right to an accrued benefit (or an incremental accrual) is then a significant factor in timing of tax liability and and movement of actual funds is not relevant.

So let's say you have a "basic" 457 for a non-profit. They allow for both elective deferrals and (discretionary) employer nonelective contributions. The employer lets eligible employees know, at some point during the year, how much employer contribution they will be receiving so they can coordinate their elective deferrals, and not go over the limit.

So let's say that for 2019, employer says they will contribute $10,000 for employee. Employee defers $9,000. All accounts are 100% vested.

The document is silent regarding any deposit timing for the EMPLOYER nonelective contribution.

Since this is an unfunded promise to pay, it would seems reasonable that there is never any deposit deadline, until payment is actually due. FICA tax is paid for 2019 as Austin mentions above, right?

As a practical matter, if the plan permits participant direction of investments, as many do, then it is unlikely that the "decision makers" will allow an unreasonable delay in actually depositing the contributions, but there's no IRS deadline, correct?

QDROphile and Belgarath have it.

Even if an employer has no obligation to deposit anything to support its unsecured obligation to pay unfunded deferred compensation, a participant might insist that the measure of her deferred compensation (often, a bookkeeping account) be credited as promptly as the plan provides, whether by its express or implied terms, and, if the plan provides participant-directed investment, according to the participant's investment direction.

In theory, it's possible to notionally credit a participant's account without the movement of money to the unfunded set-aside; in real-world practice, doing so is burdensome and difficult.