If it will help make sense further: individuals might not know their contribution limit until the tax return is done, so we have until that date to contribute to our own retirement accounts for the prior year. The same is true for an employer; the business return will give them some info upon which to base, for instance, profit sharing or nonelective contributions.

But the employee is deferring Wages. That means from Paychecks for the year. Not from a different or for a different year.

That's also why the employer's Plan needs to be established by the end of the year for the year under consideration. Otherwise, it isn't in time for that year, for the employer contribution.