Okay, let's explicitly catch this incorrect reply. It's discussed in varying forms each and every time we have a Marriott promo. It's been the same each and every time we have a Marriott promo. Let's not spread misinformation. We don't have to dissect and reinterpret the promo T&Cs language on this point every time no matter how they write it. Marriott's IT even manages to interpret and implement this point correctly. If its IT system can get it right then we can.

You absolutely do not have to complete your stay by the end of the promo for the nights within the promo period to count. This would be an absurd and highly disruptive imposition on both guests and hotel management because you'd have everyone checking out on the last day of the promo and checking back in again or - worse - moving hotels so there's no continuous stay at every one of Marriott's nearly 1.5 million rooms in 8000 properties (thanks, Google) just to ensure the qualifying nights within the promo period actually qualify. Nobody has to do this.

Stays that straddle either the start or end dates qualify for the promo for the nights within the qualifying period. Period.