SEP-IRA can be established and funded up to your businesses tax filing deadline, including extensions.
An Individual 401(k) plan can be established up to your business’s tax filing deadline, including extensions. Generally, if you want to establish and contribute elective deferrals into the plan for the year it is established, you must adopt the plan by the last day of the business’s tax year (usually December 31st). However, if you are a sole proprietor or single-member LLC, you can establish the plan by your business’s tax filing deadline and still make elective deferrals for that first plan year.
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.