I think initially they did, hence D. C. Fontana's line from "Farpoint" that Data graduated "Class of '78." Which makes the indirect references to the 22nd century and then to a much later time than the 23rd century the more puzzling.
Maybe they simply edited out the following line: Decker: "Voyager 6. This was launched over 300 years ago!" Ilia: "Incorrect, Decker Unit. 278.28234234 ---"
Publicity materials for The Motion Picture called it "a 23rd-century odyssey." So the period had already been established as Trek's era. Exactly when in the 23rd century...well, they still hadn't pinpointed it.
^The period had been established in secondary materials, but it wasn't explicitly stated in a canonical production itself until TWOK.
Rounding off, up or down large numbers is not a problem when they are given as a general description. Hence, "2 centuries" or even "20 years ago" could easily be a few above or below those figures. However, more precise numbers given by characters with a definite point to prove are less easy to ignore - why would Khan say "15 years" if he could say "16" or "18" or "20"? With the TMP situation, Decker clearly says "over three hundred years ago" so it really can't be any less than that. Unless, as someone upthread has postulated, he was just really lousy with history? This is the "old world calendar" we are talking about, not a Stardate to be seen! P.S. Maybe Voyager 6 fell into a black hole after being swept along by one of those magnetic storms that captured the S.S. Valiant?
The fix Greg Cox used in To Reign in Hell: The Exile of Khan Noonien Singh was that Ceti Alpha V's year was 1.2 times as long as an Earth year, so it was 15 local years and 18 standard years. Or just ordinarily lousy with history. Plenty of people don't remember how long ago specific things happened. And one space probe getting lost centuries ago isn't a major historical event like First Contact or the founding of United Earth, just a bit of science-history trivia. You probably remember when Sputnik 1 launched or when Neil Armstrong took one small step, but do you remember when Venera 7 became the first probe to land on Venus?
The problem with Greg's fix is that, if I remember correctly, Kirk independently uses the 15 year figure in TWOK.
^I just went over the transcript of the movie, and the number 15 shows up several times. The second line in the movie is "Leaving Sector Fourteen for Sector Fifteen," in the simulator scene with Kirk watching. Then, just after Spock gives Kirk the book, a PA voice pages Spock to say his shuttle leaves in 15 minutes. There's a "Fifteen seconds" uttered by a computer countdown during the prefix code scene. And before Kirk beams down to Regula I, Saavik advises him that General Order 15 requires an armed escort. So maybe he just had the number 15 on the brain and misspoke? (Certainly the screenwriters seems to have the number 15 on the brain.)
If it was down to numbers on the brain it's lucky WoK wasn't being made by the later Trek producers then, or Khan would have been down there for 47 years... As far as V'Ger goes, though the intent of the "What they used to call a black hole" is presumably that they don't call them that by Decker's time (not entirely an unreasonable guess as it was, IIRC, still a fairly newish term at the time), I personally like to think he just means the specific thing the probe fell into was something humans at one point thought was a black hole, but actually turned out to be something else entirely and was subsequently renamed (the "Black Star" theory mentioned above would actually fit this pretty well). The same way someone might describe Pluto as "What they used to call a planet". As with most continuity fixes, there's a bit of fudging there but is manages to explain both its strange flinging-things-across-the-Universe (rather than, say, crushing them to death) behaviour and the fact black hole continues to be used as a name for, well, black holes in all subsequent Trek. EDIT: Though thinking about it, Decker does seem to have a funny thing for strange phrases no one else uses. I think he's the only person to ever say "Star Hour" as well, perhaps it's just the case he's stopped calling them black holes and everyone ignores him no matter how much he insists his name of "SPACE hole" is better?
I always liked "collapsar" (short for "collapsed star," used in '70s SF by Niven and Haldeman) as an alternate term for black holes -- it sounds classier.
My first impulse was to say 1962, but then I looked it up and it was 1970. Though my dad died in 1970, so that sort of overshadowed everything else.
15 is still rounding off. Not mathematically, but in every day speech. The Phantom Menace came out 15 years ago. It's technically only 14, but people will say 15.