@Shannon610 you are able to open Airbnb calendar up to three years in advance.
When you first opened your account, you went though a very simplified process and you might have clicked on one year in advance. You should go though all of your settings and explore all the options you have. There are many pretty cool features on your hosting page.
If you want to keep your calendar only 18 months out and not full three years, you will block it after a certain date. I am not sure you can keep it rolling open 18 months out but you can certainly set yourself a reminder to open another month on the first of every month or something like that- however you do it on Vrbo.
Another option would be to unsync your calendar although that's a risky territory because a double booking is very likely. I often have multiple people looking at the same weekend. For example, once Chicago marathon dates are announced, everyone runs to book an Airbnb (pun intended). You seem to live in an area where different events happen as well so you are running the same risk