If somebody amended their plan document for a law that they should have amended for during a previous plan year, do they stand as a tardy amender for all of the plan year in which they amended?

I think they are considered a tardy amender forever, unless the amendment is submitted via schedule F under EPCRS, then they are given a pass for being tardy. I'm not sure what you're actually getting at.

If you are asking when the Plan is out of compliance as a result of the tardy amendment, it's the effective date of the provision that was not timely enacted. So, if the 401(a)(31)(B) amendment was not adopted timely, the Plan failed to be qualified as of March 28, 2005 (the date the new provision was required to be effective).