Does Jan 15th count towards the mega bonus
 

My current promo ends Jan 15th. if I stay 15 nights I get 25k bonus points.
I will have 13 nights, but will be checking in to a Marriott on Jan 14 and checking out on the 16. Will I make the bonus? Meaning does the last night of the 15th count?

I think yes but not sure

Previously, any stay which is in the promo period (even partially) has counted in full for the promo.

Marriott count a stay on New Years Eve, leaving the following day (year) as a stay in the old year, so applying the same logic would mean a stay overnight on 15 Jan would be counted as 15 Jan and not the checkout date of 16 Jan...

You will not receive the promo bonus because a even though you will be staying 2 nights, it will only count as one stay. To get the bonus, you will need to check into a different marriott property nearby.