after-tax employee contributions

We have a 401k plan that allows for after-tax employee contributions.  The plan sponsor is an LLC that is taxed as a sole prop.  Can the owner deposit the after-tax contributions up until his tax return due date or does he have to deposit those by 12/31?  

Thank You 

After-tax Employee contributions are subject to the same rules as 401(k) deferrals and loan repayments regarding the timing of the deposit. Meaning that they have to be put into trust promptly after they are withheld from pay.

If you are asking, indirectly, whether employee contributions can be assigned to a different plan year for 415 or ACP test purposes than the year in which they are made, the answer is no with respect to the ACP test and "I haven't researched it" with respect to annual additions.

To add to Alonzo's answer, generally a sole proprietor has to make the deferral election prior to 12/31, but the deposit can be made as late as the tax filing deadline or as soon as the schedule c is finalized.

Thanks both Alonzo and Bill. Bill's reply was more what was at issue.  The decision to make after-tax is 12/31, but for this type of business entity, the deposit can be later.

Thanks