Why do airlines change my flight time after I book?

Airlines adjust flight schedules constantly — it's called a "schedule change" and it happens to most flights between booking and departure. The reasons:

Seasonal schedule adjustments: Airlines rebuild their schedules every season (roughly every 3 months). Flights booked 2+ months out will almost certainly see at least a minor time shift when the new schedule drops.

Aircraft swaps: The airline changes which plane type flies the route, which can shift departure or arrival times to fit the new aircraft's turn time, range, or crew requirements.

Demand rebalancing: If a flight consistently runs low on passengers, the airline may consolidate it with another departure, shift it to a different time, or cancel the route entirely.

Slot constraints: At slot-controlled airports (JFK, LGA, DCA), airlines sometimes trade or lose slots, forcing time changes.

Your rights depend on how big the change is. Under DOT rules, a schedule change of 3+ hours (domestic) counts as a "significant change" and entitles you to a full cash refund if you don't accept the new time. Airlines must notify you and give you the option to cancel.

Pro tip: don't accept the first rebooking the airline offers. If the new time doesn't work, call and ask what other options are available. They may have flights at better times they didn't initially offer. And if the change is significant, always ask for the refund option before accepting an alternative.