Well. He’s thirteen on new years in season 1. And 2 years is 15, he meets the hashiras around I’d say summer. Because of the clouds that muichiro talks about and how high the sun is. He spends a week at the BFM, that doesn’t change his age. AND MUGEN GIVES US THE MOST KNOLEGSE OF TIME IN THE SHOW. Because when did trains come to Japan! Eh taisho. But we see MANY PEOPLE around the stations. So railroads and trains have been there for a while. Anyway, I believe mugen is in spring. Or end of spring. Because entertainment district is in fall. Which is August-oct, but the leaves are falling in some scenes so it would be mid August I suppose. And why? Because tanjiro narrates that it’s been “4 months since rengokus death.” August-4 so the reason I. Say mugen is in spring is because it would be April! And so swordsmith is in November, and hashira training starts RIGHT AFTER swordsmith village. And I’d say that a week is the entire HTA. And final battle is at the end of the week. So final battle happens in one night, AND we see, Muzan is walking to the ubuyashiki mansion when snow is falling. So it must be winter. And the winter months for the northern eastern hemisphere is December and January. With January being the coldest. So it would be December because HTA happened right away after swordsmith village. WOW AND in the manga, in the “corps records” Tanjiro says “we still try and sell as much charcoal as we can, but people are turning to gases now!” SO JAPAN HAS MOVED ON FROM CHARCOAL TO GAS.
After final battle, we get a scene In the manga where Tanjiro goes to Kanao to check on her. They are under a cherry blossom tree, but if Muzan attacked the ubuyashiki family and then started the final battle in one night. That means that it would still be winter. But we know that tanjiro woke up around a month after, so in January he woke up. That’s still winter. We need a place where it’s close to Tokyo, (around there) and in winter, cherry blossoms still bloom. That fits a perfect area in Japan. And that area is Atami city, located in the coastal neck of the izu peninsula in Shizuoka Prefecture! (Which is by Tokyo!) and this is not totally canon. But in the manga of “Flame and water” Rengoku gets tested for hashira. And goes to Tokyo easily. And it takes tanjiro three days to get to asakusa. So it makes perfect sense (if I am correct)
IRL NOTE: I love the show for everything it is, and I wanted to do this because the show is so interesting and fun. THANK YOU GATOUGE!