I think it may be 330 days, not 331.

Regardless, this is the practice I use. Check on Delta.com. When the day you want to fly is the first greyed-out date, you can book at midnight eastern time that night.

One recent hiccup. I was trying to book NYC-CDG-XXX. Because the CDG-XXX was the following day, it was outside the booking period. This prevented booking both legs until the following day.