sorry i meant to like (T- T ) i thought the same thing but when i re-read it says the dates where the same as the parallel world and when he woke up it back tracked like 2 days because he was in the parallel world for like two days and it took him back to his world plus his time.. but they were already in a relationship in the parallel world *cries in complication*
that was the parallel/alternate universe of them cuz there were some other students that mc didn't recognize in his class meaning in the other universe, the setting/event/people might be slightly different so that's why sen and him were dating there. i would like to think that the reason he's gone there was to trigger something in him so that he would start seeing sen in a different way. if he didn't go to the other universe, him and sen might not date at all in the current universe even tho sen did have feelings for him
So technically he didn’t go to a parallel universe he went to the future I think right