Manga Timeline
A chronological timeline of the events in the manga. Eyeshield 21 takes place in about a year (plus or minus some time for prologue and epilogue events); this is my attempt to capture the specifics of that year, including major events, Devilbat games, character birthdays, and other miscellany.
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Are these canon dates?
The short answer is: sort-of! A couple of scenes in the manga are given explicit dates (day, month, and day of the week), some are given day and month, and still others aren't given specific dates but are dated in reference to other events (for instance, a game might be said to specifically be a week after another one. Or, if we know for sure that April 30th is a Saturday, then June 5th must be a Sunday). I've my best to include additional details showing either my thought process based on canon or just the canon itself showing when a game or other event occurs. That being said, unless Inagaki himself emails me to tell me I'm genius who nailed everything, I can't guarantee that every detail here is absolutely canon.
What year is Eyeshield 21 set in?
Like most manga, the exact year Eyeshield 21 is set in is ambiguous, with no explicit mentions of a specific year. However, as I mentioned above, unlike a lot of manga, Eyeshield 21 has some specific dates for games (which is why I was able to make this timeline in the first place). For example, the Deimon vs. Zokugaku game is explicitly set on April 30th on a Saturday. This narrows down the years the series could be set in, since April 30th isn't on a Saturday every year. If we limit ourselves to years that Eyeshield 21 was being published (2002-2009), the one year that matches our given dates is 2005.
Slightly complicating matters, in chapter #77 ("Hell Is for Devilbats"), Sena rips up his plane ticket, which is dated... "Aug 29 03." There's actually no way for this scene to be set in late August, considering it takes place soon after the Deimon vs. NASA game (July 20th) and around 40 days before the Tokyo Championship Fall Season Opener (September 11th). 2003 also doesn't fit the dates we're given, so this may just be a plothole (admittedly this is a very small detail to be fixated on, so I'm not exactly blaming anyone for missing that this date doesn't make sense). (Side note: this chapter was actually published in Weekly Shonen Jump No. 13, 2004, so this doesn't match the time of actual publishing either.) So my somewhat definitive answer is that Eyeshield 21 is set in 2005.
Of course, this all assuming that Eyeshield 21 is set in our universe with the same dates and that it isn't before or after the years it was published (the surrounding years that match our given dates are 1994 and 2011. Given the level of technology in Eyeshield 21, we can safely throw out 1994 and any years before that, but 2011 is reasonable). This is also assuming that our mangaka cared enough to base this on actual year and didn't just make up dates at more-or-less random.