OAG [Official Airline Guide] is the most extensive timetable that I know. OAG is a monthly publication, and is available online via membership http://www.oag.com
Anything more than a month out is at the expense of accuracy.
Individual airlines will offer their own schedules "gratis" online, for months in advance, but can/will/do change schedules at whim.
What timetable are you using now?