The rule is:
7. MAXIMUM STAY
Return travel from the last stopover point must commence no later than 12 months after departure.
What you have a limitation of the booking tool and is a limitation in general.
AFAIK, one cannot book any flight more than 353 days out - this on QF, for AA it's 330 days.
What you do is book 'dummy segments' within the time limitations and then, when the flights on the dates you want become available for booking, simply change the dates/times/carrier for 'free' as permitted within the fare rules.
My wife and I are travelling in April and May on segment of LONE4's booked in February last year.
These were originally booked for Nov and Dec last year - we never intended to fly them at that time. In June I simply changed the flight to the dates we were after.