Tax law is harsh when it comes to most filing deadlines, which are considered jurisdictional and not subject to equitable tolling. Even one day late, regardless of the harshness of result, will block an "innocent spouse" from relief. In this case, a second-year law student from the Harvard tax clinic made an impassioned plea to the Fourth Circuit to allow an aggrieved spouse to file a petition for innocent spouse relief. The petition was filed just past the 90-day time limit. The court found that the statute plainly made the filing a jurisdictional prerequisite, and dismissed the petition.
Here is a summary of the case in the blog, Procedurally Taxing, with links to oral arguments.