after tax deposit deadline

Is there a deadline for the sole prop to deposit his 2018 after tax contributions?  He files schedule C on extension.  Can the 2018 after tax contribution be made on 10/15/19 when he files his return?  Assume, the 401(k) plan allows aftertax.  No other employees.  He is not over his 415 limit.  Just looking at the timing of the deposit.  I think rank and file participants would be ASAP and fall under the same umbrella as other employee contribution timing.   

1 hour ago, cdavis25 said:

Is there a deadline for the sole prop to deposit his 2018 after tax contributions?  He files schedule C on extension.  Can the 2018 after tax contribution be made on 10/15/19 when he files his return?  Assume, the 401(k) plan allows aftertax.  No other employees.  He is not over his 415 limit.  Just looking at the timing of the deposit.  I think rank and file participants would be ASAP and fall under the same umbrella as other employee contribution timing.   

Please clarify: are you talking about Roth deferrals as your after tax contributions? And if you are basing this question on the belief that a sole prop has until his tax return due date to make his regular 401(k) deferrals, you are incorrect.  Clarify and then I can better deal with the question.

Hi Larry, I am just referring to after-tax contributions.  I am not referring to Roth.

Given that information, I would say that if you need to have it attributed to the 2018 year, the participant needed to contribute it on the same basis as he would have for an elective deferral (401k). Of course, since it's after tax, do you NEED to have it attributed to 2018?  What if he just paid it in 2019 and you attributed it to 2019?