Can someone please direct me to where in the IRS regs it talks about the timing of sole proprietor 401k deposits?

I've found several posts regarding this, but none showing where the information was found.

I found a post that said that both partnerships and sole proprietors must ELECT to defer before the end of the plan year, but what about actually making the 401(k) deposits? I've read that deposits must be made by the due date of their personal tax returns. Is this right?

I need the reference to this.

Can anyone help me?

Thank you!

Must make election before end of year: Treas. Reg. Section 1.401(k)-1(a)(6)(iii).

Here is where I think deferral timing is discussed: Treas. Reg. Section 1.401(k)-2(a)(4)(ii) and -1(a)(6)(iv) (because deferrals before "earned income is finally determined & reported" is OK, presuming, obviously, that deferrals when earned income is finally determined & reported (due date of income tax return) is also OK.