The conventional wisdom is that you can wait until the end of the year to put a retirement plan in place since you can still get the tax benefits for the whole year. Maybe for some purposes, but not for setting up a Safe Harbor 401(k) plan.
If you want to set up a new Safe Harbor 401(k) plan for 2018, it has to be done by October 1. What’s a Safe Harbor plan? It’s a special provision of the tax law that permits owners and other Highly Compensated Employees (HCEs) to maximize their contribution regardless of how much the Non-HCEs contribute. For 2018, the maximum is $18,500 plus a $6,000 catch if age 50 or older.