Detective Conan is a notoriously long-running series -- it's been running for like fifteen years, and hundreds of cases have been solved. Conan has made friends, received upgraded technology over time (cell phones!), and has generally "kept up" with its audience.
Yet, logically, time in-universe must have passed, and Shinichi's continued absence should be more notable than it is if he's been gone for more than a year.
How much time has passed, currently, in the Meitantei Conan universe?
Technically the story plays out in real time, but realistically it is at the whim of the author.
Assuming the cases appear in chronological order. You'll notice that none of the characters ever age or even graduate. There are many discrepancies the Eisuke/Kir Arc alone has some serious and problematic time discrepancies. It starts with the Nail and Snake Arc which took place a week or so after New Years and then continues to some cases later in the Clash of Red and Black arc which climaxing in a certain person's death. Eisuke vanished around 18th to 19th of December... so chronologically speaking a year should have passed... but did it? Ask Gosho Aoyama.
My first theory is, in order to make sense of the Detective Conan universe, the stories must overlap! So the only thing that's in order is the first episode. From there, we have probably 5 cases going on at a time, and when he solves one he goes on to another. Every day he has a case to solve, whether it's one from weeks before or not.
My second theory is that even if we say each episode represented a day, which it doesn't, at least a year would have passed based on the number of episodes, and Ayumi and the rest shouldn't be in grade 1 anymore. As a result, the only logical explanation is time overlaps!
I would say that you have to look only at cases that are canon. And by that I don't mean the ones that are in the manga.
I mean canon as in something in the story line moves along and it's not just a 'guess who did it'-case.
There are only in about maybe 300 canon eps where the storyline is moved along in 750-ish anime eps. And stuff like 'The Clash of Red and Black', 'Homles Revalation' and 'Jet Black Mystery Train' are many eps long but they are only over the span of 1-3 days each time. Plus many cases are 2-4 eps but happen in a day or just overnight.
There are times where the characters will say it's been this long or that long but they are few and far in bewteen.
I would say that only about 6-ish months have passed since the start of Detective Conan.
The fact that tech in the show changes and the seasons change is simply because Aoyama-sensei the anime staff have been writing the show in real life for many years.
Although time has passed in the sense that the technology and other aspects of the setting that are not directly related to the story have progressed, very little time has passed in the main story. Occasionally, they will make references to it having been months since a previous case or other reference point, but these do not seem to line up much, if at all, and generally they are intended to indicate a time period for the individual case or arc, so that elements such as Ran's waiting for Shinichi seems to have actually been over a period of time. These indications, for obvious reasons, cannot always coincide, and so it would seem that the author decided long ago to make the story timeless. If you want a clear answer of the main plotline, so far it has been only for a few months, while the numerous side cases do not fit into this framework.
In episode 521, one of the canon episodes, the gang goes to some town to investigate a report that Shinichi solved a case wrong. That case happened "a year ago" while Shinichi was still teen-sized. Hundreds of episodes have passed since then, but that gives you a floor for the time.
I don't believe a whole year has passed, there have been several plot holes in the midst of it though. A good example would be using Holidays as a way to track time. The "first" actual Valentine episode was actually TV-only (Episode 6), yet there is another Valentine case (Episode 266-268, Volume 33 files 3-6). The Holiday that comes after it is White Day, which takes place March 14th, one month after Valentine's Day, and they make references to the past Valentine's Day as an excuse to give back gifts to the women of the show. However, the episode/volume that takes place during White Day is episode 608-609 and volume 69 files 7-9. That's over 500 episodes and 36 volumes for one month to pass in their time.
