I called and can confirm it's stays now and not nights for the Plat challenge. I could've easily done the nights, but am now wondering if it'll be worth it to break up my 3, 4, 5 night stays. Does Marriott allow check-ins, check-outs at the same property to count as separate stays if you have 24 hours in between each one? And yes, the CSR tried to talk me into a Gold challenge instead.