Date received is what ever date your 3DS has when you pick up the gift
Source
It's possible that your person had changed his system clock. This would explain the incorrect date. What's more likely however, like NobleOnion mentioned, is that the Pokemon is hacked. There are waaaaaay more hacked event Pokemon out there than real event Pokemon. It's impossible to know for sure, (if the date is really far from the actual event dates, it's almost definitely hacked).
At the end of the day it's a line of code.