What the dates mean
Since the SECURE Act, a business can adopt a new plan after the year ends and still have it count for that year, as long as it is adopted by the business's tax filing deadline, including extensions. Contributions made by the same date are deductible for the year.
Corporations have one earlier date that matters: the plan counts W-2 pay, and payroll for the year closes on December 31. If you want a higher salary to support a larger contribution, it has to run before then.
The minimum required contribution is due 8½ months after the plan year ends. Form 5500-EZ is needed once the plan holds more than $250,000, and in the plan's final year. See cash balance plan deadlines.