Manhole (2017)

In both dramas there are time travel to be able to save and win the first love.
---------------------------------------------------------------------------------------------

Both dramas deal with time travel / time slip.
Manhole(2017) - Male main lead Bong Pil time travels into the past through a manhole in an effort to stop the wedding of his lifelong crush, Soo Jin, set to happen one week into the future. whereas in
Blue Birthday (2021) - Female main lead Oh Ha Rin will revisit the past through mysterious photos left by her first love, Ji Seo Jun, who died on her birthday 10 years ago, and prevent his death. (for every time travel the person needs to pay a price )

Both have time traveling plot. Go back couple focuses on romance and comedy. While Manhole has comedy as well as some mystery. Nonetheless if you're into time travel tropes then go for it.

Same theme, but manhole is more on the funnier side.
It's more of a goofy drama with fun characters .

ML basically keep changing things in the past to get the ending he wants... FLA uses a phone while Manhole uses well, a manhole...

both are about time traveling and the two main male leads try to prevent events which are related to their loved one.

Both time travelling stories
However, in manhole, he tries to get the girl in the whole drama but in hit the top, the guy falls in love after he travels time (not before)

In Manhole, male lead tries to change the past events by travelling through manhole, while in POD 2 female lead tries to change past events. in both series changing the past reflects in the present in more bad ways.

Both have male leads which mistakenly go back in time to a certain point, even though in manhole the male lead went back to his school days and in back to 1989 he goes back to the year before he was born

the same time-travel drama because of their first love both have humors in it though Proposal Daisakusen is more of a slice-of-life/love-story drama nonetheless, both stars real-life buddies, Jaejoong in manhole, Yamapi in ProDai.

Both are time-travel drama because of their first love and the guy is trying to change his destiny and make the girl love and marry to him.