After-tax contribution deadline

My understanding is that the deadline for a participant to elect voluntary after-tax contributions is the due date for the employer's tax return (as opposed to traditional or Roth elective deferrals, which must still be elected by December 31 (even though they can be funded later).  Can anyone point me to authority either supporting or correcting that understanding? 

(Note:  I'm specifically asking in the context of a solo 401(k) plan for a sole proprietor.)

Thanks!

After I posted this, it occurred to me that the real issue in this situation is probably the 415 rules.  Per Treas. Reg. § 1.415(c)-1(b)(6)(C), an employee contribution must be made no later than 30 days following the close of a limitation year in order to be treated as an annual addition for that limitation year.  So, I think that is really the outside limit (not the tax return deadline).

You are correct - seek and ye shall find!

Wanting to return to this, please.

Whether a sole proprietor or owner of an entity taxed as an s corp or just a w-2 employee that works for a company that allows after-tax contributions: can that person decide in January to make a contribution and do so prior to 1/30 and have the amount count as 415 for the prior year?

I can't find anything specifically saying yes or no. We know the deposit doesn't have to happen before 12/31. But does the decision/election have to happen before 12/31? We know it doesn't have to be a payroll withholding, so I'm thinking that it doesn't have to happen before 12/31.

Agreed.  There's no deferral election if employee just writes a check to the plan, which must happen under the 415 timing regulations that BTG referenced above if they want to count it for prior year.

5 hours ago, Belgarath said:

Agreed.  There's no deferral election if employee just writes a check to the plan, which must happen under the 415 timing regulations that BTG referenced above if they want to count it for prior year.

One thing in our basic plan document indicates "The Employer may collect Participants' After-Tax Employee Contributions using payroll reduction or other collection procedures."

So, I think the check gets written to the Employer and the Employer deposits it in the plan? Although they could have it written directly to the plan.