I've done this to Sydney.

You can buy a regular fare and ticket the return date. Once you've flown the outbound segment, call UA and cancel the return segment. The return ticket remains valid for a year from the departure date. You can call up and change for the date you want later on and pay the change fee of $250 if I remember correctly.

Be aware that you need the same fare bucket to be available. I'm guessing they would charge you the difference in fare if it's not. I try to avoid the cheapest fares and typically buy W, so it hasn't been a problem. I found all this out from the 1K line.