My theory is in that building, a group of students got caught up in a fire, earthquake, or some type of disaster and die. I think the female lead will be among the victims, which is the reason he timeleaped. To save her and anyone else caught in the accident. It would be almost poetic. The man who is in an era where he shouldn't be in saves the girl who shouldn't exist in the future. They end up getting married and we skip to their wedding where we see him, his wife, and all the classmates he saved.