Reunited Worlds (2017)

both have time travel elements with a chance to write their wrongs
both have Kim Jin Woo starring in it

Both have time travel elements with a chance of redemption

Both shows have Jung Chae Yeon in it ... for sure it will be good

These two dramas are similar when it comes to the main leads being wrongfully accused of deeds they didn't commit yet took the blame/fall for. How one wrong accusation ruined their life. Finding out the truth over that event is also a similar theme in both drama.

The main lead dies and returns again not having aged a day old from when they left. The leads try and figure out what happened leading up to their death and try to fix what they can.

The two dramas are very much alike. What both have in common:
- dead people
- love triangle... kind of
- a loaded chief who is the boss of the heroine
- mix of comedy, romance, supernatural, mystery and crime

Longing Heart and Reunited worlds are similar in certain points like how they are both time themed drama. Both without much focus on the details on how time was bent
—The school setting is important for the plot
—precious familial ties and moments. The main leads understand how their actions or absence affected their surroundings
—high school first loves

Both cooking themed
Both female lead actors are working hard in a restaurant
Both are perfecting the "vongole spaghetti"

the main lead is from a fantasy world.
he also has superpowers
the plot is interesting

In both dramas...
-Female and male leads are in an older women younger man relationship with both guys being in high school
-Second male lead also has issue with his father
-Male lead and second lead's father have issues

The main characters in both these dramas are the eldest in the family, warm-hearted, protective of their loved ones and often put their happiness aside. They both faced unexpected tragedies, but are given a second chance to amend unresolved issues and restore their name.

Both dramas have the same feel although I haven't finished Reunited Worlds and I don't wanna spoil 9 Seconds (since it's a short series) - both have time travel/time stop theme