They are only going to count what they had collected by 8pm. Everything collected after that time, backdated or not, is an invalid vote.

That's why the Trump legal team is suing in federal court over Republican poll watchers being blocked from observing the count of mail-in ballots. That is key. Giuliani and company are alleging that Democrat operatives blocked Republican poll watchers from observing BECAUSE they were mixing in the post-election day balance with the rest, as well as separating them from their original envelopes thus severing their tracibility and making it impossible to verify signatures. Signature verification as a requirement was struck down by the Pennsylvania Supreme Court (along with the 8pm election day deadline). This is setting up to where the only fair and equitable remedy the U.S SCOTUS can provide is to 1)discount ALL the hundreds of thousands of mail-in ballots not observed by Republican poll watchers or 2) vacate Pennsylvania's entire election because it is so tainted by fraud and unconstitutional effects of the PA SCOTUS. Samuel Alito ordering all post election day ballots be separated from the others was a signal from the Supreme Court 1) that they have the number of Justices required to agree to hear the case from the Trump team and 2) that post election day ballots and the handling thereof will be a huge factor in the case. If Dems are found to have violated Samuel Alito's order to separate post election day ballots they will be found in contempt of the highest court in the Land. Not trying to be overly optimistic, but I think these are good signs and they give me hope. The SCOTUS was deadlocked on PA 4-4 before Amy Coney Barret, she'll be on the bench this time when they hear the case again.

Keep in mind, the "no votes after 8pm on election day" isn't just about counting ballots from voters who procrastinated on sending their mail in. What it's really about is counting ballots generated by Democrats at 4am to rig an election and mixing them in with the rest of the votes while not allowing Republicans to observe what was going on.