I have a few questions about the ending:

1. Was the guardian of the building the one who instigated the time travel? After all, she appeared to have knowledge of both the past and present.

2. How was there a time loop? Surely, once Shin Jae Woo returned to the present, the time loop closed; there wouldn't be a never-ending loop of meeting with Kang Ji Hoon. I didn't get that part of the time loop?

3. Was it a parallel universe rather than a time loop?

4. When Shin Jae Woo visits his mother during the 2 hours before the "supposed" departure for their trip, she appears to be the same age in 2007 as in 2022.  Weird!  Or, did I mis-read the scene.

5. If Shin Jae Woo really wanted to save Kang Ji Hoon there were a myriad of ways that would not have endangered them, e.g. anonymous (possibly exagerrated) report to the police about the kidnapper/murderer (once he found him again)? Calling the police slightly ahead of the incident reporting a kidnapping ... So many ways.

I'm sure I'll think of others once I watch it again on the weekend.

1. I think the guardian of the building was some sort of a guardian of the time: past, present and future. That's why she knows everything. But I don't think she sent Jae Woo to the past.

2-3. But there wasn't a time loop, was there? I mean, there are only a few weeks in 2022 when Jae Woo doesn't "exist" and there are two in 2007. The time flew just the same and there weren't overlaps.

4. I think they didn't a great job with the make-up. I didn't look at it so carefully, though, and I don't know if it hints about something else...

5. Yes, there were many but I'm glad that the authors kept the past exactly the same as in our timeline. Another solution could be that the three boys went to take little Jae Woo from wherever he was and accompanied him home making another route. And even if they bumped onto the kidnapper, they were three young men against one man with a knife.

~

I just realised replying to your questions that there's a few weeks hole in 2022 where there's no Jae Woo. That's probably what Ji Hoon meant that JW could meet him every 15 years? You have any idea? The letter mentioned a third meeting between them... 

"I just realised replying to your questions that there's a few weeks hole in 2022 where there's no Jae Woo. That's probably what Ji Hoon meant that JW could meet him every 15 years? You have any idea? The letter mentioned a third meeting between them..."

That's what I meant by a time loop.  The end of the series is the whole process starting over again, as in some kind of loop, which doesn't make any sense.  In his letter, Kang Ji Hoon mentions meeting every 15 years to spend just the 3 weeks together, as in the same thing will happen again, and again, i.e. the time loop I'm referencing.

I'm not sure what you mean by there being no Shin Jae Woo in 2022 though. Do you mean the time in January when he's in the past?  This doesn't allow him to meet Kang Ji Hoon again though after he failed to save him - unless there is a time loop, but the timeline is now closed. He has returned to 2022, and time continues in a straight line (as understood in western cultures). The "guardian of time" as you referred to her is nowhere to be seen, so how can he travel back, again?

Not sure when the 10-year-old Shin Jae Woo disappears in 2007 either, as he doesn't go anywhere in time.

If i understood it right, there is no time loop.

 After Kang Ji Hoon still went to save Shin Jae Woo, he told the young Jae Woo "to come back to him". There are two things changed, the young Jae Woo met the mean guy who told him that Ji Hoon will die because of him, and second, Ji Hoon telling him "to come back". That Jae Woo will realise later, latest when looking in the mirror, that that mean guy is himself. Now with the message of Ji Hoon telling him to come back, that new Jae Woo is now actively looking for the dormitory in 2022 to go back in time and meet Ji Hoon again (the first meeting again for this Jae Woo).

Thats how i understood their "fate".

Who knows, the two might make other choices this time, since the starting point is different with Jae Woo not being confused about seeing Ji Hoon again, and maybe both, the young Jae Woo and Ji Hoon survive, so the young Jae Woo can spend more time with his Ahjussi Ji Hoon