Tom Clancy's "Dead or Alive" plot point - Minor Spoilers

Cafe Society
1

So i’m reading the latest Tom Clancy (plus co-author who probably wrote most of it). It’s OK so far, some eye-rollingly strawman depictions of people his politics disagree with. But nothing we haven’t seen before.
As i say i’m still reading it so no spoiling the second half of the book please. Here is what has been niggling away at me however. It’s a early plot point that whatever crimes (technically or otherwise) the Campus may commit. They have 100 blank presidential pardons signed when Jack Ryan was president which they can just fill in the details if they face any legal problems.
So a President can issue pardons for crimes which occured years after he has left office, i am neither American or a lawyer but i can’t believe those pardons could be valid.

        This can't possibly be legal can it?
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I think it’s less a question of whether it’s legal, or whether whoever’s in charge when their crimes are discovered lets them get away with it. Even if it’s technically legal, I can see it not being accepted simply because we don’t want Presidents and Governors handing out Get out of Jail Free cards that are valid no matter what atrocities you commit, and even if it isn’t technically legal, having the previous President’s blessing might sway the current President’s decision to pardon you.