It's 180 + 10.
Meaning, you can start booking 180 days out from the first night of your stay, and then you can book up to 10 days beyond that.
So, if your arrival date is 11/14/2015 (180 days from today) then you can book from 11/14/2015 through 11/24/2015.
If your trip is longer than 10 days, you'll need to wait for that window to overlap any day beyond 11/24/2015.
Hope this helps!
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