Time Machine (2021)

The leads travel back to the past in both dramas (although for separate reasons and through different means i.e. photographs in blue birthday and cell phone in first love again)

it has time travel, they're first love, investigation, parallel words, cellphone like a portal, school drama, youth drama

It's the same as blue birthday. It has time travel content. Travel through time to save someone. And also fix love.

Time travelling grew up in the same neighbourhood. The male lead travels many times to change the female lead's destiny in First Love Again. In Lovely Runner, the female lead travels back to change the male lead's destiny.

in both dramas the main leads are in different time lines and connect through a cell phone to try to change the past and the future. In this drama they are teenagers in the past and young adults in the present/future.

same theme drama : fantasy, traveling time , another dimension of world.
both ML is genius in physics.
the difference was sisyphus the myth is the action thriller romance drama. meanwhile the FLA is just a romance drama

This drama is also about time travel. The ML of future time contacts with past version of FL through a phone. He fixes all of the messes by contacting with FL.

This drama is also about time travel. ML used a phone to time travel and fixes all of the messes. ML travels time eclipse multiple times throughout the phone.

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

A young girl receives a letter from her future self, giving her and her friends a chance to change the future and live without regrets.

The male lead from First Love Again is the second male lead in Someday or One day.Both dramas switch timelines throughout the drama .

I haven't seen First Love Again, but if this is your thing it sounds like there's a lot of similar elements, such as magical cell phones and trying to change the past 10 years ago.

The concept of time travelling; in DDMS the FL makes a time travel back to here high school days, reliving, recreating and cherishing the moments she once missed, whereas in FLA the ML lead first time travels back to his school days and then keeps travelling in parallel universes in his present self trying to change the past so as to return to his original time space. (the 31-year old ML).

Hello, Me! is very different from FLA, except for the fact that there's time travel in both.
In Hello, Me! the FL's 17 year old self time travels to the future (the present where the story takes place), where both the 17 year old FL and her 37 year old self try to change the way she had lived for 20 years, boasting her self confidence, reviving old friendships, and igniting romance.

Both dramas are time-travel romance drama.
In See You Again, FL dies in a car accident and ML uses a camera to time-travel back in time, hoping to save her.