That information is outdated. Since the merger with Continental in March United is using the old Continental booking engine and you can book United saver award flights 335 days out. You can book standard awards (2x mileage) at 337 days out but I don't know why you would want to. You can also book partner award flights 337 days out if the partner supports it.
Of course, United may or may not open saver award availability for a specific flight at 335 days, but that is the earliest they currently open them. United allows you to book one-way for 1/2 the roundtrip award miles so if you think the availability will be snatched up fast you should consider booking the outbound first, then the return when it becomes available. The only problem with booking them separately is that if you have to change your dates you will pay 2x the changes fees (since you made them as two separate bookings, unless you are one of the top status level United flyers in which case changes are free).
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