A defined benefit plan or cash balance plan must be set up by the time you file your business return (including extension) for the related tax year. For example, let’s assume you have a calendar year-end, you can adopt the a 2020 plan as late as September 15, 2021. But realize that the investment account must be set up and funded by this date as well. So, you will need to coordinate with your administrator and custodian well in advance of the deadline.