Yup 180 days before your check in date. You can do upto ten days from there. For the last 4 days of a two week holiday you need to do the ADRs at 179,178 and so on.
I think from memory the lines opened at 12pm uk time and available online from that time as well. Might be easier to do it over the phone as you need a different reservation number to make the bookings. I have done both phone and online and found on the phone to be much easier.
Think it's 6am Eastern, so usually 11am UK, except for the week between US and UK summer time ending (be 4 hour time difference between 28/10 and 4/11 this year).
I think I was hitting Try Again on my browser until about 11.04am on my 180 day mark .