I only want to explain about the story in my own perception. It can be wrong but I think I will give a simple review about this movie since it's very short, with only 3 main casts and several settings, but it's totally grab my attention!

Yes.. it is a very confusing to understand the ending.. However... the meaning of the whole story is very deep. And I can tell the writter is very brilliant!

Now we go to the story:

Both Yuki & Sakai here have the same perception and situation toward their life.. which is they feel life is boring.. the world hate them, and they hate themselves too, so they have no idea how to change and if the change is necessary.
In short, they give up to their life since they only take daily routine and it feels like their time is totally stuck.

But Yuki suddenly find her dream that is a good looking yet passionate guy: Shota, who later on lead her to change her life.
Through her meeting with Shota, she learn how to change her life to be a likeable person, and to be a better version of herself each day, so she can be loved.

This is a story that describe about this simple illustration in our life:
"when you feel lonely and hate your life.. and when you don't want to change to be a better version of you, then your day is actually just repeated the same (since you only do your daily routine)."

So here..in this movie, when Yuki's time keep repeating the same is actually symbolize about people life.. like us, that sometimes can feel stuck in our daily routine, so we keep repeating our days, as if nothing has change.

The broken MP3 in this movie is a message. Yuki keep using it and never try to fix it eventhough its broken since she think its functionally still be fine.
It is the same with her life. She think there is no need to fix herself or do any changes on it.

When she realize that she wants a "tomorrow" in her life, and she feel that life is better with many changes and all of the uncertainty, she give her broken MP3 to Sakai.. means she is ready to change her daily and face her life in different way of thinking.

She then reveal everything to Shota, and showing him how much she wants a new day to come for her. Then yes.. finally, her day starts to move again, that simply describe about her life is start to move again.
Meanwhile Sakai is still being trapped in his own time, since he think it's okay to repeat the same day as it can still make a better version of himself day by day.

So.. don't think too logically about how and why there is time travel, or those magical thing has happened in the movie. Simply put it as the reflection of our life.
Daily routine in most of people life, is actually no better than the time that is not moving at all, and just keep repeating everyday.
Try to make changes and face your challenge everyday, then you will see the tomorrow has come.