Date of Plan Adoption

For a plan effective 1/1/22, under SECURE, does the document need to be signed by 12/31 or would the tax deadline suffice?

SECURE extended the deadline to adopt to due date of the tax return with extension.

Though if there is a 401(k) component, you can't make the 401(k) contributions retroactively.

Also, if it's a defined benefit plan (DBP), where a valuation needs to be done to determine the contribution that will be deposited by that tax return due date, you'll want to get the document done and signed far enough in advance. I recommend anyone considering a DBP retroactive to 2022 to put their tax return on extension for the extra time but make the decision and get the document done in the first or second quarter of 2023.