Yes the chapters at the beginning are in the future, when all the boys should be in college or college age, but Jian Yi is having to redo high school. Every now and then, we will get chapters flashing forward to this time again (when Jian Yi and Zhang Xixi are at a class reunion, and when He Tian meets up with Mo Guanshan in the alley behind his job, and they go off on Mo's scooter, for example).
You can see in the newer chapters (newer than what's on this website), it seems to be winding up to He Tian disappearing as well. Maybe he takes Jian Yi with him and they go into gangster hiding, iunno.
I can't find it now, but there's a timeline floating around. Right now the boys are in junior high, I think their last year. In the timeline I can't find, someone has parsed out all the junior high chapters and you can see (at least, last time I saw it--by this point it might not fit) that everything has happened in less than 19 days so far. Go back and start counting, you will see how many chapters fit into one day, and if you take it as there are no gap days in between except where specified, it might still fit. If so, the title might imply that we're counting down to the end of the junior high stuff,
presumably to when Jian Yi disappears
??? Or until the end of the school year maybe, I don't know how their school years work.
I'll keep my eye out for the timeline. If anyone else knows where it is, let me know. Pretty sure it's on the yaoi-blcd tumblr but buried in there somewhere.
Edit: Found the timeline.
Part 1 https://yaoi-blcd.tumblr.com/post/154573849130/19-days-timeline-up-until-16-dec-2016-by
Part 2 https://yaoi-blcd.tumblr.com/post/174727304980/19-days-timeline-part-ii-up-until-25-may-2018