Can I establish a Small Business Retirement plan for the previous tax year?
SEP-IRA can be established and funded up to your businesses tax filing deadline, including extensions.
Starting in tax year 2020, an Individual 401(k) plan can be established up to your businesses tax filing deadline, including extensions. However, If you are looking to establish and contribute elective deferrals into a plan for any given tax year, you must adopt the plan by the last day of the business’s tax year (usually December 31st).
The deadline to establish a new SIMPLE IRA for any given year is October 1st unless your business came into existence after October 1st in which case you can establish the SIMPLE IRA as soon as administratively feasible.