I feel there must be a thread covering this but I can't find it.

I found a decent non-refundable price at the Fairfield Manhattan/Times Square and I booked it this morning. Looking again tonight I found a good rate on Trivago at the New York Marriott Marquis and submitted a LNF request. But I don't want to cancel my existing reservation until Marriott approves my LNF change.

My Fairfield rate says "Changes to your reservation are not permitted. Please note that you may cancel your reservation for no charge until August 16, 2017." What time on the 16th do I need to cancel by? Is it 11:59 Hotel time? I can't seem to find anything other than the date.