Same here - on my 60 day mark, I could book that day and the following 9 days out. I have not seen any official information on this, but could it be 60 + 10?
It did take quite awhile - I was on-line at 6am. Every time I tried to save a day, the website came up with a system error and threw me off, but the reservations are there. Just took longer to keep going back in. It took me about 2 hours or more to book 10 days of FP+.