Foreign 401k

A foreign company sponsors a 401(k) plan for some of its employees who are US citizens living in a separate foreign country.

415 limit deadlines are based on when a company's US tax return is due correct?

So how does one determine the deadline for non-safe harbor contributions if the company sponsoring the 401(k) doesn't have a US business tax return to file?

I'm hoping to avoid looking through treaties. But it is 2020...

Are you sure they can legally sponsor a 401(k) Plan as you have described?

I think I found the answer:

26 CFR § 1.415(c)-1 - Limitations for defined contribution plans.

(B) Date of employer contributions. For purposes of this paragraph (b), employer contributions are not treated as credited to a participant's account for a particular limitation year unless the contributions are actually made to the plan no later than 30 days after the end of the period described in section 404(a)(6) applicable to the taxable year with or within which the particular limitation year ends. If, however, contributions are made by an employer exempt from Federal income tax (including a governmental employer), the contributions must be made to the plan no later than the 15th day of the tenth calendar month following the end of the calendar year or fiscal year (as applicable, depending on the basis on which the employer keeps its books) with or within which the particular limitation year ends. If contributions are made to a plan after the end of the period during which contributions can be made and treated as credited to a participant's account for a particular limitation year, allocations attributable to those contributions are treated as credited to the participant's account for the limitation year during which those contributions are made.