If a hotel wants to have an indisputable deadline, it just takes a specific date, time, and time zone. These details should be part of the reservation process, should be on the conformation email, and should be in the reservation details when the reservation is viewed online.

Let's consider the current words, such as: "You may cancel your reservation for no charge until May 31, 2014 (1 day[s] before arrival)."

Wording with a date but no time reasonably suggests being able to cancel any time on that date, until 11:59 p.m., based on the time zone of the hotel.

The part about "before arrival" is there to say whether it's the day before your check-in date date (or 2 days or 3 days or whatever). It's not to establish a time.