I am staying onsite for 2 days this weekend, then will be off-site after that. At the 60 day window, I was only able to book for the days I will be on-site, however, I kept checking and as I was 60 days out from each of my off-site days, I was able to book my FP+'s for each of those days. I had to do them one day at a time, as each 60 day period arrived. Hope that makes sense.
I guess what I am trying to say is..... you won't be able to do all days right at your 60 day mark, but from my experience, you also don't have to wait until 30 days.
Good luck. Hope this helps.