Originally Posted by
fumje
No it's definitely not until midnight the next day.
There often is some grace period of about an hour or even a few, but I haven't found it to be reliably a certain amount.
I bought a ticket yesterday and decided to price the same itinerary out again today. I noticed that the fare went down by $500 per person. So, I canceled the ticket and the website said that I am within the 24-hour window. After purchasing the new tickets at the cheaper price, I decided to take a look at the timestamp of my emails. It turns out that I canceled the ticket 14 minutes after the 24-hour window. I guess the grace period does exist! Phew!