As of July 2016 when I asked their customer service staff, they said end of following day. Book today at 3 PM, have until 11:59 PM tomorrow to cancel. Apparently their systems are not designed to check up on you for the hour or minutes, just the day itself.
Unless that has changed since then I stand by what I was told.
Has anyone else contacted Air Canada directly about this within the last 2.5 years to give an updated and accurate response?