Let’s assume that they make their own arbitrary rules on that.
The question is can they tell you that your vote cast within the voting timeframe will not be counted? Is your right to vote a lesser right? Lesser than any state’s right to invalidate it? Which other right is lesser to a state right? Free speech, right to assemble, what?
That’s the only thing the SCOTUS can rule on. Not the timeframe. That’s Congress’s job.