How much of a "grace period" does someone have before incurring a 4971(b) tax? I understand that many people do not always recognized that one must fund a money purchase plan within roughly eight months, and so they do not make a contribution until just in time to make the deadline for deducting the contribution on their tax return (and since many of them have their tax returns filed until six months due to an extension), which often leads to a 4971(a) tax.

So, say the plan sponsor (in this case the taxpayer, not the plan document sponsor of the volume submitter or master/prototype) does not make the plan contribution for a money purchase plan until October 5. Obviously, he or she ends up subject to a 4971(a) tax but does "a grace period" prevent the taxpayer from standing as susceptible to a 4971(b) tax?