I think that this explains why joining 100+ challenges hampers my reading--I feel pressure to finish the book not too long after I started it. And it doesn't count if I don't finish it in the year that I started it. So I'm declaring my own 100+: My goal is to finish 100+ books this year, irrespective of when I started them. If I continue that from year to year, then it really doesn't matter when I started them.
That's the plan. I should probably go read something now.