FP+ question when staying off-site then on-site

[cmjohannes]

My family and I will be traveling to the World in August. We will be staying offsite August 9-16, then staying onsite at the Poly August 16-21. I know my FP+ booking window will open at 60 days from August 16th (the day I check into the Poly), but I have read elsewhere in the forums that once my 60 day window opens, all days prior to 60 days will open as well, effectively making it possible to book my FP+ for the off-site portion well in advance of the 30 day mark. Is this accurate? Can anyone provide any insight/knowledge into this?
Thanks in advance!