I say "no".
Do you live in a place that requires an emissions (no CEL check-engine light) inspection? And would you need same by August?
P0420 is what is referred to a "Catalytic Converter Efficiency". It is basically referring to how well the catalyst is converting hydrocarbons, carbon monoxide, and NOX.
The P0420 code can be caused by a whole bunch of things, including a failing converter. If that's the original at 180K, this is probably way past it's prime. It could also be a failing downstream "secondary" or "post-cat" oxygen sensor ... which tend to last less mileage than the catalyst they monitor.
However, some long highway runs (>30 miles) might clear it out a bit.
If it were me, I would clear (or have your local auto parts store) clear the P0420 code, and see how long it takes (or even if) it comes back. Twelve miles a day (especially if that's 2 six mile trips) is not sufficient heat to get that thing catalyzing. It's basically a furnace, which uses the unburned hydrocarbons (gasoline) along with something called SOC "Stored Oxygen Capacity" to further burn and convert everything HC/CO/NOX into water and CO2. Hence the term "three-way catalyst". But, it needs some time and heat to really get pumping, and get everything back in balance, and clear any smorf, dreck, and crud (highly technical) off of the honeycomb matrix of platinum and rubidium and expensonium. A really hot hard run can do wonder in restoring catalytic converter efficiency. They also sell magic snake-oil elixers you add to the gas tank that claim to hurry this process, but I can't confirm if any or all of them do any good. My uncle said a quart of transmission fluid and a pint of moonshine, but I can't confirm that either. Seems like a total waste of two good drinks... (actually, that's a joke. Don't do that. The drinking part. Or the gas tank additive part.)
There are no performance or wear or reliability issues with a P0420 or any other catalyst failure. Your vehicle will run fine. You may be producing emissions (pollution) that is out of specification. Such a thing is for your morality/cost ratio to judge; certainly not mine. There are several "Kitty went bye-bye" vehicles in my fleet, but they are honestly used off-road only (race cars).
So... that's a lot of wind from the old man, but I think the first thing you should do is get the code cleared and take her out for dinner a few highway counties away.
There's no problem whatsoever in driving it until August, in fact August 2020 or more. You can't "hurt" it more. Having no CEL may improve resale value - although in my never-humble opinion a vehicle like that needs to be driven right into the ground. It doesn't owe you much and if you decide not to replace the cat, it won't cost you much either. The Xterras (Xterrae? I don't know the plural) I've worked on over the years have been super-solid and reliable. If I had one I wouldn't part with it until it neeeded a trans, engine, or the driver's seat fell through the floor!