For the non-spoiler part:

1. The time travel is straightforward and easy to follow. No advance time travel theories. No time travel gymnastics. Nothing crazy. Pretty much linear.

I strongly advise finishing the entire series first because the show itself explained the linearity of the time travel. Again, it's simple and easy to follow.

However… this is the SPOILER part.

They made a mistake on two things:

1. They dated the murder of Kwon Min-Ju on 1998-10-13.
2. When Han Jun Hee arrived in July 1998, her dialogue was she realised she only had 3 months to save Min-Ju.

Yet,... read the rest.