Just in case someone else is wondering, I called in because it's relevant to me as I had originally wanted to fly a mRTW September 2020 buuuuuuut.
So if you book a flight August 31st 2019, your latest OUTBOUND has to be August 31st 2020, and your latest completed flight has to be August 31st 2021.
Now to calculate if the extra stop is worth 450CAD in change fees to allow me to do this in August-September 2020...