The New ‘Game Of Thrones’ Season Starts In April, But What About That Book? George R.R. Martin can’t type fast enough to keep up with HBO.

HBO confirmed Monday that the sixth season of “Game of Thrones” will premiere in April. For fans of the show, this is presumably fantastic news, but for fans of George R.R. Martin’s long-running book series — the basis for the show — it’s bittersweet, to say the least.

Martin has published five of the seven novels planned in the series, “A Song of Ice and Fire.” And while the HBO program is trucking along right on schedule, Martin is not. The sixth season will be based on what would be in the sixth book, “The Winds of Winter,” and that has not yet come out. The showrunners are essentially out of printed source material, but they do know the gist of what happens in the forthcoming novels. Martin has said he wants the next book out before the next season, but there’s a strong probability that fans will see the ending of the story on television rather than on the page.

I’ve crunched the numbers behind the varied ways people have tried to predict when “The Winds of Winter” will be published, and earlier this year, I looked at one of the best analyses of Martin’s writing pace, developed by one of his closest observers. The writer — real name Jeff, well-known in the online “A Song of Ice and Fire” fan community as “BryndenBFish,” particularly for his sophisticated analyses of the book series at his blog — projected that “The Winds of Winter” would come out in late 2016 or early 2017. In other words, definitively after the sixth season.

But now that we know when the sixth season will start, I caught back up with Jeff to talk about whether Martin could possibly finish before the premiere. And Jeff said it’s possible, if unlikely, that fans could still be able to get their hands on a copy in time, mostly because of the wild production schedule for books like Martin’s.

“While the average turnaround time for a book to go from ‘manuscript submission’ to ‘published and on the shelves’ usually ranges from four to six months,” Jeff said, “Random House hustled and published ‘A Dance with Dragons’ seven weeks after they received the final manuscript.” (The accelerated publishing schedule for these huge books is a double-edged sword: It’s great for fans who want the books yesterday, but it raises many questions about how much careful editing Random House really gives one of its star authors.)

One reason it’s so hard to predict the release of “The Winds of Winter,” compared with the previous entry in the series, “A Dance with Dragons”: There’s little information coming from Martin and his editors about how much of the book he’s finished. Based on details disclosed by Martin’s editor Anne Groell and by Martin himself at HBO’s season four premiere, Jeff estimated that Martin has about 200 pages written for “A Dance with Dragons” that were not used in that book and can be rolled over to the new one, and 168 known pages of new manuscript that have been turned in.

Jeff reckons on an April 24 premiere date, based on HBO’s scheduling of a forthcoming show, “Vinyl,” and thus estimates that Martin has to submit a book by Feb. 23 in order to make the finish line — this is the minimum amount of time necessary, assuming an accelerated production schedule last seen with “A Dance with Dragons,” for a book to hit store shelves before the premiere.

The long and short of it?

As Jeff put it: “He’s going to have to submit something in 90 days or it’s likely he won’t publish in time.”