Unconstitutionality has a time limit?

Or so the Pennsylvania Supreme Court declares:

The Pennsylvania Supreme Court on Saturday rebuffed a long-shot election challenge Saturday from one of President Trump’s top boosters in Congress, balking at his suggestion that it throw out every ballot cast by mail or designate the state’s legislature to decide who won the state.

In a unanimous decision, the justices declared that U.S. Rep. Mike Kelly (R., Butler) and the seven Republican plaintiffs in the suit had waited too long to bring their lawsuit alleging that the 2019 law, passed by the state’s GOP-controlled legislature, which created no-excuse mail voting in the state for the first time was unconstitutional and “illegally implemented.”

Instead of filing it shortly after the passage of the bill, which was required in the statute, they waited until their candidate lost to challenge the mechanism by which some 2.6 million Pennsylvanians voted this year, the court wrote in a terse, three-page order.

I very, very much doubt that this is a surprise to the Trump team. Remember, wait two days…. It’s a bit strange, though. If the law flies in the face of the state constitution, then it presumably continues to do so regardless of when it is formally challenged. Does unconstitutionality really have a time limit?