Not sure about AA, DL definitely used to do 11:59pm a day later, but not sure if that is still SOP. UA, back before the CO merger and DOT requirement also used to allow 11:59 next day (and also used to additionally allow a hold, allowing you to essentially keep a refundable flight for 3 days). That was more important, IMO, in those days because most cheaper fare classes had a $150 (or $200) change fee, but these days, aside from BE, there is no change fee and so you essentially can use the full value (assuming your not the 1 x per year traveler) for another trip.