Not all airline booking systems have the same booking horizon. 364 and 330 are two comon ones but I know of airlines with 180 and even 90 day horizons.
United used to use Apollo (330 day horizon) signed a deal to use Amadeus (364 days) decided to merge/buy Continental and then to use their Shares system (330 days) and buy themselves out of Amadeus and close Apollo. Air NZ use Amadeus (364 days).
But don't be lulled into a false sense of security. Airlines can and, more to the point do, change their schedules. Just because your booked on UA1111 at 1215 now doesn't mean you won't end up on UA1117 at 1207 in the end and if you've chosen your seats don't assume they'll have protected that seat selection when they reschedule so keep on checking.