6:00 am (EST)
On my 180+10 day, I was able to book our arrival night dinner online at 6:00 am.
However, I had to wait until 7:00 am (EST) to call Disney Dining & book our other ADRs.
It seems, w/ the new system, you can book online exactly 180 days out, but the system is not recognizing 180+10 & you have to call.
However, a helpful DISer posted a link to the previous online dining system which is apparently still live, & some people have been able to book all their ADRs on their 180+10 date by using this link.