I had $20k spend in late December (after my statement date) and $20k spend in early January (before my statement date). I was expecting 40/5*2 = 16 night credit in 2019 but only got 8 night credit (plus the 5 nights for simply having the card). That means the $20k spend in late December counted for 2018 not 2019.
I timed my purchases specifically to reach 100 nights in 2018 and nothing more, but sounds like I ended at 108 nights instead, so I am "losing" the 8 nights which I would have preferred to count in 2019 not 2018. Had I known that they were using December 31st as the cut-off rather than the statement closing date, I would have timed my large expense differently.