I think the drama had an ok concept that was made so much worse by the execution/writing of that concept. The original idea was probably two pages long and basically the rest is a massive uninteresting filler.
I've just finished watching it and if you asked me to explain what happened in X or Y episode I wouldn't be able to, because there is so much nothingness that it automatically got erased from my mind as soon as I watched it.

I think one of the main problems is that they decided to make FL and her younger self complete opposites and under a very specific set of circumstances with the accident so it's not something many people will relate to. I could feel connected to the lost present time FL but not to her younger self nor the dynamic they had going on later. If there had been more introspection into the first part of FL's character I would have found it more interesting. I also still don't understand how having her past self there helped her to get anything she managed to get. Her younger self is selfish, spoiled and kind of annoying. The dynamic they have later of mother-daughter also I think made it quite a lot worse because by their actions and words they pretty much worked as such and made you even believe it at times and personally it's not how I would treat my younger self... but then again, my younger self was not that childish nor it was that different to how I'm now so I think the relationship would probably be more like we're sisters or something in those lines, what I think kinda makes more sense and probably would be more true to most people, especially considering her younger self was 17, not a 6 year old.

To this we add an overabundant amount of scenes of side characters that add nothing to the main story nor are they that interesting. FL's sister appears multiple times yet I don't know much about her, also she was quite bad in the beginning yet later is just sweeped under the rug. Anthony's scenes bored me sooooo much, I don't understand what the point of him being there was. It was just a filler since his story was not even that related to hers, not even in the end. I kept thinking there was going to be some connection that justified him there but there wasn't. I don't think his story was bad per se, it's just that it's not worth 16 episodes of a few scenes per each one. He's main character but why???? He really adds nothing to the story. His arc could have been done in like one episode and done.

I always find it unrealistic when a main character starts working in a place and turns out they knew someone in there from the past, like how small is your city? Here it's not only the best friend, but also Anthony and Han Yoo Hyun. Too many coincidences and, at least when it comes to Han Yoo Hyun specifically, very sloppy reasoning.

I enjoyed the character of Han Yoo Hyun but it was very forced how his father kept him out of the business because "he's not interested" but wouldn't even give him a chance when he was the one showing an interest, it was just pushed again just to drag the story more until the last episodes. It also got tiring he kept calling FL "old woman" even when they were closer.

There is no romance here and it's only in episode 14 that Ban Ha Ni mentions having butterflies when interacting with Han Yoo Hyun... so you're just told because scenes we had none and maybe we got a few looks from him that let me know he likes her and him saying it to Anthony, but her to him? zero! She looks like a rock in that sense the whole series, even at the end when they say they're together, the chemistry is zero and I'm again being told and not shown (that kiss was appalling, especially for people in their 30s, what was that!!!!??? hahahaha). This show shouldn't be classified as a romance, it's not.
This brings me to the fact that had they focused a lot more in the romance throughout the series they would have had enough extra content to not have to have so many fillers of people walking here and there, or talking very slowly about uninteresting things and so on.
The main plot is, well, very sloppy. It was ok and interesting in the beginning but it's kind of forgotten for most of the series till the later episodes that it's brought back. Once they know what the mobile phone message with the days meant and they were doing nothing I started rolling my eyes, especially considering I was already fed up with the show at that point. They look at the phone and see D-7, they panic for 5 minutes and then they have some nice meal and go somewhere and then they show D-6, and then panic again and two seconds later they're eating beef. It was so difficult to take this seriously. Also the father gives that criptic message through the old phone "quickly quickly, you're running out of time", but considering where she had to go in the end to go back that it's nowhere related to anything she would have been able to know on her own and "quickly quickly" but the dream message was only received one day before the expire date. Bad writing, that's all.
I'm going to nitpick here as well because I was also rolling my eyes at the whole dream/past sequence, you're in a field of dead grass and you find a lily of the valley there eventhough they mostly grow in woodland, humid shaded areas? I was laughing, even more when in the tunnel sequence they find one and they're holding it with the hands (it's very toxic and can produce rashes) until I realised it was a plastic one, what fair enough, but I mean, there was no reason whatsoever really that it couldn't have been another flower the one picked for the whole idea instead of a lily of the valley...
This also brings me to how utterly useless that phychic was, I get they wanted to make the quirk of him being a modern one what was fine, but he would give half messages even when he knew what was happening. You can't tell people something is very serious and life and death and urgent and at the same time withhold information... he only appears twice in the show eventhough at one point he says to FL that he'll be looking for a way to make her past self back, next scene he appears that matter is not discussed.
Not sure why this is also labelled as a comedy, I only laughed once, and that is when Sung Woo is giving some food to past Ban Ha Ni and his mother, FL's sister, has a go at him. It did make me chuckle, but other than that I didn't find anything funny.

The whole drama in the company was ok but kind of cliché and I don't like how they dealt with ML's aunt in the end, she's kind of forgiven and it's not only what she has done with the company but the fact that her son never addresses the way she treats Oh Ji Eun. She had been said horrible things multiple times and yet nothing happens. In fact I don't even know the point in terms of plot of Oh Ji Eun getting pregnant since we didn't even know until that second that she had been trying nor she has a change of character at any time that makes her stronger or more confident or stopping taking that kind of treatment from her mother-in-law.

Points go for FL having freckles and having scenes when later on she has started using makeup for work but she's still shown with the freckles at times when she's supposed to not be wearing makeup, I thought it was a realistic touch I appreciated.

Overall is just boring and dragging and writing is not great. The message is there but you don't need that many episodes to tell it.
Considering as well that there isn't really a progression in terms of work, romance or much else of anything, this could have prefectly worked as a movie and it still would have given the same message.