I'm sure Class of '78 means the same thing to Data and Riker as it does to us. That is, a date marker of some kind.
Not saying he graduated in 2278, but it could be something like Stardate 22378 (~May 18, 2345), or maybe a simpler stardate system in use in the 2340s or by the Academy. Or maybe it was the 78th year following an Academy rededication in 2268 (and thus, Riker and Geordi were Class of 87, Picard was Class of 57, Saavik may have been Class of 18, Wesley would've been Class of 103 or 104). Or the 178th year following the standardization of the Academy year circa 2168 (a few years after the official incorporation in 2161, which itself was a couple decades after they were graduating their first classes of troops although that may have been a different Starfleet Academy).
It wouldn't make sense for even early TNG Data to lie to Riker about such a simple fact checkable thing, especially after Riker admitted to reading all of Data's records (then why are you asking if the rank is honorary, Bill?). But therein lies the rub. "Class of 78" might be a code that shows that Data actually attended the Academy and knows some of the shorthand lingo they use for identifying the graduating Class. Riker knew the official records (that we saw much later) stated Data attended from 2341 to 2345, and when Data confirmed using a non-file placeholder (year?), Riker was convinced of the sincerity of Data's Starfleet record, and went on to question him about his potential superiority complex (which Data confirmed).
And for me the answer is simply that the United Earth government sometimes changes sumbols like the Earth flag, the Earth coat of arms, the Earth anthem, etc., fin order to please various voting groups perhaps. And that includes sometimes changing the official Earth calendar, or at least the official Earth calendar era. A clendar era is the moment in time that the years are counted from.
If you go to the Wikipedia article you will see in the upper right corner a box labeled "2021 in various calendars" . You will note that the current year aD 2021 has different year numbers in many different calnedar eras, and they differ by tens, hundreds, and thousands of years.
The UFP government may also soemtimes change the calndar and/or calendar era used.
So when Data said that he graduated by Starfleet Adacemy in the class of '78, he meant that he graduated with a class in ayear number ending in 78. But when he told the revived dead people in "The Neutral Zone" that i was 2364 iin their calnedar, he meant that their calendar was a different one than the one used at that time. Obvsiously when the revived dead people returned to Earth, their fame helped the people who wanted to have their calendar made the new official United Earth Calendar and dates given in later seasons would thus dates arr ccoridng to the calendar era used by the frozen dead.
And to me it seems like the only logical way to reconcile Data's Class of '78 iwith dates givien in later seasons is to deduce that the official united Earth calendar was changed to that of the era of the revived dead persons.
It drives me nuts that Data's "class of '78" line is written off as a continuity error, yet McCoy's age being 137 in 2364 has been written in stone, even though it makes McCoy too young during TOS.
Data said that Mccoy was 137 years (of an unspecified type) old according to official Starfleet records. And maybe Data used some sort of official Starfleet years that might have been slighlty longer than Earth years, making McCoy somewhat older than 137 Earth years and old enough to agree with his age in TOS.
Even if Data used Earth years for McCoy's age, we may wonder how do official Starfleet records record a person's age. Do they just subtract the person's birthdate from the current date? Do they calculate the number of years that the person's body has been alive and biolgically functionings and omit counting the years that they might have been dead or the years they might have skipped over thorugh time travel? Do they calculate how much or how little a person might have aged during suspended animation?'
In a science fiction story a person's age can be much higher or lower than the number of years since they were born.
If anything, it makes him a bit too old to be best buddies with Jim... That is, unless we go by the backstory that finally gets explicated by JJ Abrams, of McCoy being a graduated MD first and a Starfleet officer only after that fact, thus sharing some Academy history with Jim (even if the good doctor never took the class on Starfleet jargon such as 'dunsel').
Timo Saloniemi
Are you claiming the opposite of what JohnnyQuest037 said, that if McCoy is only 137 in "Encounter at Farpoint" that will still make him too old to be Captain KIrk's friend?
Is there some sort of legal limit on how different the ages of two friends can be? Do you claim that some psychological process makes it impossible for someone to become friends with someone more than a few years older or younger than themself?
I find it hard to understand what you are claiming, and so I can't decide whether to agree or disagree with it.