The "Class of '78" line was because the show was operating on the Spaceflight Chronology timeline at that point.
Whatever the chronology the show was using then, the line remains in a canon episode and was actually said in canon.
Therefore, it seems logical to deduce that in the Earth calendar used in the era of "Encounter at Farpoint" the date when Data was graduated from Starfleet Academy and was commissioned in Starfleet was '78 of an unspecified century.
Now look at "The Neutral Zone":
RALPH: What year is this?
DATA: By your calendar two thousand three hundred sixty four.
Data's words do not mention whether Ralph's calendar is also Data's calendar. But by specifying that it is "your calendar", not "our calendar", and not "the calendar", and not "The Earth calendar", Data seems to be implying that the calendar has changed at least once since Ralph died.
And it seems pretty clear that Ralph's calendar was not the calendar that Data used in "Encounter at Farpoint".
As I said in post number above:
The year of "Datalore" should have been sometime between about '91.000 of one century to '01.998 of the next century.
So obviously a year can not be both 2364 and from '91 to '00, or be '01, in the same calendar.
Since publicity in the first season said that it was happening in the 24th century, I am assuming that it happened between 2301 and 2400 of that calendar.
So if the year was between 2391 and 2400 in Data's calendar, and 2364 in Ralph's calendar, the year one in Ralph's calendar should be between about 27 to 36 in Data's calendar.
If the year was 2301 in Data's calendar and 2364 in Ralph's calendar, the year one in Data's calendar would be about 63 in Ralph's calendar.
If a United Earth government is formed, it may sometimes change its flag, coat of arms, anthem, etc. to satisfy various pressure groups that advocate various symbols that they consider best for United Earth. And possibly the United Earth government would sometimes change the official United Earth calendar to satisfy various pressure groups.
So presumably the United Earth changed the official United Earth calendar at least once while Ralph was preserved in the capsule. And my guess is that when Ralph, Sonny, and Clare returned to Earth they became celebrities via the closest future equivalents to tabloids, television, and the internet, since they were from even farther in the past than Khan's people would have been if they returned to Earth.
The fame of the living dead people used to Ralph's calendar would have been used by pressure groups advocating for the use of Ralph's calendar to push to have Ralph's calendar decreed the official United Earth calendar. And it looks like they eventually succeeded, since in later seasons of TNG, DS9, and VOY, dates of contemporary events, when given, were consistent with being given in Ralph's calendar.
The creators of TNG might not have intended to depict the United Earth changing the official United Earth calendar in the early seasons of TNG, but that is what they ended up depicting in the first seasons of TNG. That is what the first few seasons of TNG depict. That is in the canon of
Star Trek.
How many dates mentioned in any
Star Trek series mentioned AD or BC, CE or BCE? According to my count, few enough to be counted on the fingers of two hands - possibly on the fingers of a single hand of some species.
And IMHO it would be very foolish to assume that any
Star Trek where the dating system is not specified is in the AD/BC or CE/BCE system.
