Originally Posted by
Concerto
It depends where you buy it. If you get it from Air France Germany as I did, and 24 hours is up at midnight, then you'll be stuffed after 20h because the German office closes then.
If you find yourself frequently needing to cancel tickets within 24 hours, perhaps you should either a) buy the ticket at a more convenient time of the day, or b) wait the full 24 hours - and if you still want the ticket, then buy it!