I am not sure why this is not bookable at the 180 mark, but it always seems to be extended. It is very popular and we enjoy it. On our last trip I noticed that they had added ice cream which was a nice touch and it wasn't some soft serve machine. It was good vanilla hard ice cream and it was pretty good. I must say that I felt really sorry for the guy that had the job of scooping the hard ice cream out of the tub. He struggled for every single scoop he scooped. Even with the reserved area you need to get up by the railing about 20 minutes prior for the best spots.
Keep checking daily online to see if it opens up, because it can sell out quily once it does. Hopefully in the near future this will be bookable at the 180 day mark.