The mechanics of time travel are presented in a clear and uncomplicated way, avoiding complex theories or convoluted plot devices; it unfolds in a largely linear fashion. It's best to watch the whole series before analyzing this aspect because the show itself clarifies how the timeline operates—it’s quite simple and straightforward. However, there are some inconsistencies that came to light. Specifically, two dates appear to be incorrect: the date given for Kwon Min-Ju’s murder was 1998-10-13, and when Han Jun Hee traveled back to July of 1998, she stated she had only three months to save Min-Ju. But in Episode 10/11, Jung In-Kyu reminisced about a Christmas he shared with Nam Si-Heon and Kwon Min-Ju. If Kwon Min-Ju had died on 1998-10-13, that memory wouldn’t have been possible. The actual date of Kwon Min-Ju's murder was 1999-10-13. Han Jun-Hee possessed Kwon Min-Ju’s body for roughly fifteen months—five in 1998 and ten in 1999, not the three months indicated by her dialogue. I don’t know how this significant detail was overlooked, or why a Christmas scene was filmed and incorporated into Jung In Kyu's recollections. If Kwon Min-Ju died in 1998-10-13, then that Christmas scene wouldn't have happened; conversely, if the Christmas scene is accurate, her death must have occurred on 1999-10-13.

Source on TMDB