What is the deadline to set up a plan?

Defined benefit plans must be set up before you file your tax return (including extensions). But the latest date is Sept 15th. Remember that drafting your plan documents will take approximately one week and then you will need to open your investment account and fund it. So you should allow yourself at least a month to get it all accomplished. That's why we have an internal setup deadline of August 15th.

In order to take advantage of the employee deferral on 401(k) plans, you must have it set up before December 31st. If you have employees, you likely will need a safe harbor 401k plan and the deadline to set up these plans is Sept 31st.