episode 2 & 3 done, ep 1 & others to come.
I'm assuming least amount of days passing; every short time skip is "the next day" unless reason to assume otherwise. If there's reason to assume otherwise, whether that's because it's definitively the next day, or because it's definitively more than a day skipped, I'll make a note of it.
1995
Yu Dukhoon, Euijeong's father, retires from his position as commissioner of the SMPA. [Episode 3]
1996
DAY (1) JUNMO'S UNDERCOVER OPERATION BEGINS
Night scene. Junmo goes with cover story and watch to meet Gicheul at Sikgaek; Gicheul gives him his card and offer to meet. Gicheul is wearing a greyish/light suit with a dark shirt underneath, Jungbae is wearing a dark suit. [Episode 1, Episode 2]
DAY (2)
Night scene. Gicheul and Jungbae visit Taeho’s father to do a background check on Seungho. Gicheul is wearing a dark striped suit with a dark shirt underneath. Jungbae is wearing a very blue shirt, his suit jacket (lightish-colored) is off. Unlikely they both changed suits in the same evening as the previous scene, which pushes it off a day. [Episode 2]
DAY (3)
Daylight scene. Junmo shows up at gang HQ with leather jacket on, fights everyone and convinces Gicheul he’s not in it for the money. Oh Kyungjin is wearing a grey suit. Gicheul wearing peachy-colored shirt and dark suit with stripey-diamondy designs on it, Jungbae wearing dark suit and dark shirt.
Jungbae and Heesung put together the briefcase of money. Same day, Gicheul wearing same shirt but has taken his jacket off, Jungbae wearing same clothes too. Heesung wearing dark suit jacket/white shirt. [Episode 2]
DAY (4)
In Gangnam street, day into night shot. Junmo disperses flyers searching for Taeho’s killer, making sure to be spotted by Gicheul’s men. Oh Kyungjin is wearing a dark suit with a dark bluish shirt underneath it, dot-patterned; he could’ve changed his shirt and suit the same day but a weird thing to do for such a minor character randomly. Probably meant to show a time skip. [episode 2]
DAY (5)
Day again. Heesung takes the money to Busan dude and contacts Hando about the ambush.
Night shot now, Junmo calls Euijeong at a payphone. Gicheul, Seo, Heesung and Jungbae eat at Sikgaek. [episode 2]
DAY (6)
Hando gets the go-ahead for the ambush from Song Donghyuk; Gicheul invites Junmo to the “party” in Taeho’s honor. Day into night — the fight, then Gicheul invites Junmo into the gang. Song Donghyuk decides to have Gicheul killed. [episode 2]
DAY (7[8])
Daytime again. Junmo introduced to the gang in HQ and joins it formally.
Night, “walking dramatically down the street” scene. Junmo is different clothes than they were in the previous scene, no leather jacket but instead a suit with a plain white shirt. End of the same day, or perhaps another day?
Perhaps the same night — Junmo and Yongdae on patrol. Yongdae is wearing a shirt with scribbly blue lines going across it horizontally. They break up a fight, eat street food, break up another fight while Dohyung drives by. [episode 2]
DAY (8[9])
Night. Same patrol montage, but Yongdae is now wearing a vertically striped shirt. It’s at least one day later. Junmo is still wearing a plain white shirt. [Episode 2]
DAY (9[10])
Day, Junmo is at Undercover operation HQ, no suit jacket, wearing a white shirt with blue stripes and he’s trying to get some sleep before patrol that night. [Episode 2]
DAY (10[11])
Night shot, on patrol. Junmo is once again wearing a plain white shirt, Yongdae is wearing his vertical striped shirt. [Episode 2]
DAY (11[12{13}])
Daytime shot. Euijeong is wearing a tan jacket with a thick-vertically-striped shirt greyish/greenish underneath, not a button-up.
The same day, still daytime. Junmo asks Yongdae why he’s staying with him, Yongdae explains he used to live with his parents but they moved away. Junmo asks Yongdae to wash up, Yongdae says he did that this morning, Junmo says that was yesterday morning.
Euijeong, same outfit, calls Junmo’s colleagues because Junmo didn’t call since yesterday, and finds out he’s on leave for “asthma.” Daytime: Junmo now at payphone, in dark suit with subtle stripes, hears Euijeong’s message asking to meet. He’s wearing his plain white shirt. Yongdae is wearing a blue/purple/jaggedy-design shirt.
Night. Euijeong and Junmo together, her eating now in the same tan jacket, buttoned, different shirt underneath (you can see it more clearly when she rushes out of the eatery later—it’s a white button-up.) But, she could have changed her outfit the same day/after she got off work; Junmo’s wearing the same suit as in the previous scene (definitely bought by Gangnam Union as she doesn’t recognize it). Same day or another day? Uncertain… [episode 2]
Junmo has been undercover for anywhere from 11 to 13 days
DAY (12[13{14}])
Euijeong visits her mother at Yeongdong Severance Hospital.
Evening. Gicheul stands by the window thinking of Euijeong. Junmo watches him; no suit—plain dress shirt. Detective Cheon gets a series of faxes asking for background on "Kwon Seungho" and on Euijeong. Junmo at the payphone; looks like same shirt with dark suit jacket on top. [episode 3]
DAY (13[14{15}])
Early morning. Section Chief Lee meets with Detective Cheon and is given fresh American donuts. Cheon brings the info to Gicheul while it's still morning. (The men are changing and saying "good morning"). Junmo tells Gicheul he's never heard of "Park Junmo." [episode 3]
Cho Chansik confronts Section Chief Lee, making him eat one of his ill-gotten donuts; it is still fresh—therefore obviously the same day. "Seungho's" fake information was prepared sometime in advance. [episode 3]
At Undercover Operations HQ, Junmo comes up with an idea to put pressure on Gicheul by having a feint attack on the Gangnam Union by police officers pretending to be from a rival gang. [episode 3]
Night. Junmo and Bae Yongdae drive around and see the "attackers." Junmo follows them to Gangnam Union HQ and fights them, seemingly saving Gicheul's life and getting a minor head wound in the process. The Gangnam Union go to the clinic where Junmo is patched up. Jungbae tells Youngdae to get Junmo a cell phone. Heesung tells Yongdae to get Junmo and himself new suits. The same night, still in his bloody suit and with his forehead bandaged, Junmo tells Dohyung that the plan worked. The policemen who were in the "attack" celebrate with dinner. [episode 3]
DAY (14[15{16}])
Junmo walks into Gangnam Union HQ in his new suit (solid light brown/dark tan, with a white striped shirt underneath) while the men are fixing up the trashed office; he is given his room in the HQ near Gicheul. Junmo calls "Euijeong" (the Undercover Operations HQ) and informs them of his new cell phone. [episode 3]
DAY (15[16{17}]) The 15th*
*see DAY (17[18{19}])
Junmo and Gicheul talk in Gicheul's office. This must be a time-skip because now Junmo is wearing a dark blue pinstriped suit with a plain white shirt underneath. Instead of staying at Gangnam Union HQ for dinner, Gicheul takes Junmo with him on his visit to Yeongdong Severance Hospital to visit Euijeong's mother. [episode 3]
At the same time, Tsuyoshi Oyama (Yakuza) and Song Donghyuk (Busan Gang) discuss getting rid of Jung Gicheul, while next to a painting of a house with a dog. [episode 3]
Back to Gicheul visiting the hospital. Outside, he takes a call from Kanemoto and OKs Song Donghyuk's death. Song Donghyuk dies in the same suit he was in when he planned Gicheul's death: the entire plot until his execution at Kanemoto's hand seems to have taken only one afternoon. [episode 3]
DAY (16[17{18}])
Junmo wakes up in his bunk in Gangnam Union HQ. As Gangnam Union prepares for the fishing trip, Gicheul tells his inner circle that Kanemoto took care of Song Donghyuk (who they think was behind the recent "attack.") [episode 3]
Junmo steps out into the main room in his dark blue pinstriped suit, this time with his striped white shirt underneath, and tells Chief Seo "good morning." The fishing trip takes off. Junmo asks Oh Kyungjin how often Gicheul goes fishing; Oh Kyungjin says "about a couple times a month." Then he says, "why, you want to go out or something?" He continues by telling Junmo that he should stay at his post. [episode 3]
Gicheul and the others go to the boat; Jungbae stays to keep lookout. [episode 3]
Junmo, in the same shirt—with the sleeves rolled up, no suit jacket—is cleaning HQ. He asks Jung Byungwoo when Gicheul is expected back. Byungwoo answers, "sometimes, he stays one night. Sometimes, it's two nights. He's not coming tonight." Oh Kyungjin invites the men out drinking, but Junmo stays behind. He takes pictures of items in Gangnam Union HQ as evidence. [episode 3]
DAY (17[18{19}])
In Undercover Operations HQ, Junmo shows the printed photos to Cho Chansik and Dohyung. This must be the next day; he is now wearing his dark suit with a white and blue wide striped shirt underneath. [episode 3]
At the Tokyo Metropolitan Police Department, slides showing information about Song Donghyuk are showing as the Japanese police explain he went missing after entering Japan on the 15th. Because Song Donghyuk came up with his plan to kill Gicheul and died the same day facing Kanemoto, this sets where in the timeline "the 15th" should go fairly specifically. [episode 3]
A gang-member is selling Gangnam Crystal, but is taken and murdered by Manager Cho. This seems to happen in one afternoon—he comes to the vintage store on his bike, gets the meth and a new jacket, leaves with the new jacket and wears it into the wooded park where he's then cornered. [episode 3]
Afternoon. Jungbae, Gicheul, and Heesung get ready to drive back from the fishing trip and find out that the flashlight left behind by the "attackers" is a police flashlight. This would make the fishing trip have lasted one night. [episode 3]
Night. Manager Cho meets with Jungbae behind a school and drives away with him. He is attacked by Gicheul and Heesung, who are following in a car driven by a woman. She stays behind while Gicheul, Jungbae, and Heesung take Manager Cho to a deserted place. In Gangnam Union HQ, Chief Seo wakes Junmo and tells him to come with him, and they drive to join Gicheul, Jungbae, and Heesung. Gicheul then tries to make Junmo shoot Manager Cho to prove his loyalty, since the finding of the flashlight has cast doubt upon him again. [episode 3]
Junmo has been undercover for anywhere from 17 to 19 days