I'm in California and when I made mine it was at 6 AM ET or 3AM PT. However, it was very time consuming and only allows you to make 10 at a time. I had some problems and ultimately ended up calling for the few I could not get online and that ended up being at 7:30 AM ET when I was finally done with online reservations.
Personally, I think the online system is great for making those hard to get ressies asap and for checking up later but the phone ressies were much quicker. Although online does have the benefit of getting an e-mail confirmation...