The SECURE Act which allows established 40(k) plans to convert their plans to a safe harbor (ADP only) at any time during the year did not change the rules that apply to establishing a new plan with safe harbor contributions.
A newly established plan (other than a successor plan) must have at least three months left in the plan year to be a safe harbor plan. For example, for a 12/31 plan year end, the elective deferral and safe harbor contributions must be effective no later than October 1.
The only exception to this rule is for a newly established employer. After the employer comes into existence, the new employer may establish a safe harbor plan with less than three months left in the plan year.