So I read a oneshot a couple of years back and was looking for it again. It had a female MC who woke up with strange older man in her bed/room who claiming to be her husband who time traveled from the future. She also has a neighbor who's a bit younger than her who has a crush on her and is always going over to her house to borrow books (he's like really into science or and her dad is a scientist??) And the older guy like to tease him a lot, but in the end we find out that they're actually the same guy. Basically after making his past self feel bad, the guy and the girl confess their feelings for each other and the guy from the future says he's actually engaged to her and he was worried that their marriage would fall through so he had to go back in time(not quite sure about that reasoning).