Note that when you are offered an event, you have to pick up the event from the event person in the PokeMart (this is actually via Wi-Fi). That could be any date, while the time the event was actually implemented on the day was within the timeslot you have talked about. And, I did not obtain the Celebi this year; it was last year during summertime (when I had my free time). So it pretty much fits.
The date doesn't really matter, to tell the truth. I've RNGed the Celebi from the event. I could have made the date 2099 or something bizarre if I was lazy enough, lol. So pretty much, it's legit.