Through watching the movie, I deemed it great to watch!
First and foremost is due to the visual graphics and animations it contain: being wonderfully displayed and laid out. The music, sounds, and the voices are all fit for the movie.
Second, is the genre of the movie being concentrated on Sci-Fi and romance, tackling the concept of parallel worlds or what if's, arising from the things we imagine what future we are embarking if we do something different than what we did. Though the timeline or the span of the time in the movie is relatively short, (1 day), it produced a unique plot about what we would do if we were able to correct our mistakes even if it means it is going farther and farther from reality (which may be the price for being able to wish to change one's past to meet a desired ending).
Third, it lets you think. The movie actually lets us perceive what it means to wish to have a chance to go back to the past and be able to change it so that you may reach your goal.
For me, the ending was that, when the firework ball was shot up to the sky and shattered to bits and fragments, the fragments shown some what if's of each person's mind and see its essence if it were to come true. Not only that, the weird and made up world by the main character came back to normal, except for the fact that they are still there, having their time together. It means that what he has done, changing his past to get what he desires, became that of the real world. Thus, was the reason for their absence at their class the next morning, since they are still together from getting away from the parents and the friend. (PURELY JUST SPECULATION BASED FROM MY UNDERSTANDING THOUGH HAHA)
Though the explanation or emphasis for the reasons of why it has occurred is not deliberately justified, that's why a number of watchers are not able to comprehend a great movie with symbolism and deeper meaning.
Overall, I did not regret watching the movie and had a heartwarming feeling of having to see the relevance of thinking about something that you would like to do but did not, and imagine what it would be if I do it.