You're making a gross assumption.
It's about as gross as assuming that Kirk's rank in TOS was "Captain".
Btw, this page outright states Christine Chappel is a ncom too.
So perhaps you could go and correct the page? That's why it's constructed as a wiki.
Maybe he's referring to star date and not Anno Domini (or it's "CE" retcon)?
...Or perhaps he prefers the Omicron Theta calendar? But personally I'd like to do away with the idea of year classes altogether, considering that there is no set length for studies. Yeah, it might be interesting to hear which year Data received his Ensign pip - but not that interesting, as this would tell the listener nothing about when and with whom he took the classes. And Starfleet doesn't appear to believe in a schedule when it comes to promotions, so there'd be little to be learned from being told the number of years between that Ensign pip and the Lieutenant Commander ones.
Timo Saloniemi