The Employer contribution for the Solo 401(k) and the entire SEP amount (as it is considered an Employer contribution) is often not determinable until the business return is prepared (unless the client clearly made more than enough...), so that explains why the employer amounts have a due date of the due date of that tax form. And even though the Employer Solo 401(k) portion can be deposited after the end of the year, the plan needs to have existed prior to that year end and not by the due date of that tax form.