About the time leap, manga skipped a part from the LN in chapter 11.
After going back to the present Seiji learned that he and Hiiragi chan broke up before graduation because he was tired of her. He then leaped back to the past, and during lunch he absent-mindedly told her he was tired of karaage. She ran away in tears thinking he was talking about her and he realizes that this was the misunderstanding that caused them to break up, so he was able to patch things up.
@nazarmf if you already like the manga, it is. But keep in mind, as others already said, this is almost entirely fluff slice of life,so don't expect an elaborated plot.
@gameriuxlt - First of all, this is literally, and I DO MEAN literally a slice of life fluffy romance story. If you're reading this expecting some deep, or hell even slightly interesting storyline, besides their interactions with each other, you will be severely disappointed.
Second, and you can't be blamed for this, him going back in time has very huge relevance, and there is a point to it. The manga just hasn't got there yet. Is it some crazy stuff like Steins; Gate? Hell no but it is indeed relevant.
So far, the biggest thing that has relevance is the fact that, he actually transitions back in forth between the present and future versions of himself in an attempt to change the future. At the moment, the two biggest reasons are to get Haruka's mother to accept their marriage. She is that standard high-class bitch that thinks unless the mc is someone "important" he's not good enough for her daughter. The second is to help Haruka's father. Her dad owns a fairly large company and in the future, it goes bankrupt. This, in turn, makes it so Haruka's sister needs to live with them, parents no longer live well, etc. In fact, him saving the company is his best bet to get Haruka's mom to accept their marriage.
@jonhstriker - There are only two instances that I felt were unnecessary and annoying.
At one point, Haruka's mom forced her to go on a date with some nasty ass son of a CEO for another big company. It all resolved with the help of Seiji and funnily enough, Haruka's dad. The worst thing that happened is he put his hands on her thighs rubbing them, but that's it. The second thing is, Seiji got amnesia for a little bit and his sister took advantage of it by saying they were in an incestual relationship.
Other than those two that pretty much resolve itself as soon as they popped up, nothing "bad" happens it stays fluff through-out.
literally no reason why the author made him travel back in time, he acts like teenager shy and whatnot, he never used any of the future knowledge or anything. Literally it would've made 0 difference.
The only reason i could think and even that is stupid one is that the author didn't want to make teenager to date teacher so he made teenager be from the future or something. IDK