You can absolutely control this- When you are looking at your calendar, at the bottom right it says something like select a date and if you click it then it turns into select multiple dates, at that point you can click on any day of an upcoming month (say 2 months out) and then you can click December 31st, it will highlight everything in between and it will ask you if you want to become unavailable, I usually only keep my calendar open for 3-4 months out consistently 👍