For those who are wondering about the end, I've been Reading the WN and seen the manga raws.
Have you seen the movie Butterfly Effect? This story is like that. Every time Seiji goes to the future (present time actually) something is off, like he and Haruka get together but her parente don't accepted or they accepted, but he's with Natsumi (Haruka's sister) instead and he doesn't ver along with his sister anymore
Everything indicates that Seiji and Haruka broke up in the last year of the school and this starts with Sana discovering about the relationship. The WN still don't translate to this point, but it seems that Seiji broke up with Haruka because he hurt his sister's (Sana) feelings and he doesn't want to distance from her. Sana is the one who convices Haruka to reconciles with Seiji. Both Seiji and Haruka marries at the end.