Thanks, but neither of the IRS links you posted say what you say in your reply. There is so much bad information on solo 401(k)s on the internet that I have learned to look solely at IRS documents.
IRS Pub 560 (page 3 of the pdf) does say what you say, but my question was:
"I see from the IRS website that the deadline for making solo 401(k) contributions is my filing deadline (that is April 15), but it is unclear to me whether I can file earlier (say February) but then wait to April to make my 401(k) contributions."
