Booking an advance purchase rate online, one is confronted with this text:
Advance Purchase rate, prepay in full, non-refundable if cancelled more than 1 day after booking, no changes, see Rate Rules

The rate details also include this (for a booking made on 8/21)
Please note that you may cancel your reservation for no charge until August 22, 2015.

So if the booking is made at 9AM on 8/21, until when does the free cancellation period extend? Is it until 8.59 AM on 8/22 (i.e. a 24-hour free cancellation window) or is it until 11.59PM on 8/22 (i.e. a calendar day) or some other time?

I called the Marriott 800# but got different answers and had the distinct impression that the phone agent was not very sure of her response.