Late PBGC Form 500

Plan is terminating 12/31/XXX0. Files 5310, but much later (more than 180 days after termination date) files Form 500 with PBGC. IRS approves 5310 submission, with 12/31/XXX0 termination date in XXX2. PBGC comes back and says termination date must be 12/31/XXX1 because Form 500 was filed more than 180 days after termination date.

How do we reconcile this? There may be 4019a)(26) issues with the XXX1 year due to the additional of additional employees. What is the cure for filing Form 500 late? Do we need a XXX1 valuation?

I'm assuming the termination amendment/resolution was a freeze & term since you are worried about an additional year of service?  The freeze portion would not be affected by a change in the termination date.  The PBGC has a complex because they're not a regulatory agency, but they're allowed to write their own rules; and it's usually best to conform to their timelines as much as possible.  The designed termination date is just a counting tool, the Plan itself doesn't terminate until it's 0 - $0.  Modify the term to conform to the PBGC and file an "amended" Form 500.  You may or may not have needed a '21 valuation to begin with, your actuary will direct you there.