It’s not a special syntax. It’s just a slot number in the very near future. (But not so near that you cannot finish your work – including repairs to f’ups – in that time frame. I’d probably choose one to three days if you expect to perhaps need some answers from these forums or so.)
Slot number in the past will not work. The minting will get rejected, … because it’s policy says, it’s not allowed anymore.
BTW: The cardano-cli transaction build will need an --invalid-hereafter parameter with the same or an earlier slot number. At least it barked at me, when I didn’t include that. Makes some sense to ensure that the transaction will not go through if it violates the policy.