I think 2012164 may mean “day 164 of year 2012”. That would be June 12th, which seems a reasonable date for this point in the story (though we’ll have to assume 2012 was a typo for 2011).
The 164th day of the year is on June 13th (or June 12th if it’s a leap year, which 2012 was and 2011 wasn’t). June 13th, 2011 is only a few days away from when this chapter takes place.
June 12th is certainly a good date to be involved in the Arc that (I assume) has chapter 13.4.
Oh! That makes a lot of sense. It’s kinda impractical as a way to convey a date to someone who isn’t used to thinking of days like this (i.e. most people), but I guess at least it doesn’t lead to any “which is the month and which is the day” confusion.
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