3/25 is the 180 day mark for 9/21. If you are staying onsite, you can call on 3/25 and make all of your ADRs for your entire vacation. If you are NOT staying onsite, you can only make ADRs for 9/21 on 3/25. You will need to call each morning for each next days ADRs (call on 3/26 for 9/22, 3/27 for 9/23, etc). The 180 days is from the FIRST day of your vacation- you can then make ADRs for up to the 10 days following that, if you are an onsite guest.
Hope that helps- it can be confusing, I know.