This article is from the Stardates in Star Trek FAQ, by Andrew Main zefram@dcs.warwick.ac.uk with numerous contributions by others.
Objection: in the fourth season ST:TNG episode "Family", Picard visits his
family in Labarre, France, Terra. This being stardate [21]44012.3, this
theory makes the date 2367*01*05, which should be winter in that part of the
world. In the episode it was very clearly not winter.
This is a strong objection. In fact, it seems likely that the writers simply
made a mistake -- the production team worked on the basis of stardates
divisible by 1000 being the beginning of the year, though they never got that
information onto the screen. This system assumed that that was the case
primarily for simplicity. Other things being equal, this consideration would
be overridden by the on-screen evidence. However, in this case other things
are not equal.
In TVH:STIV (stardate [19]8390, 2286-10-09), the `Whalesong' probe visited
Terra. It was observed to cause severe atmospheric disturbance at the time.
It has been conjectured that it had more severe climatic effects than were
readily apparent. The novel "Probe" (entirely non-canonical, of course) sets
out some of these ideas. (It's also a good novel.) It is conceivable that
the effects of the probe included a shift in Terra's orbit, or permanent
climatic change, such that in January 2367 the atmospheric conditions in
France resembled a 20th century summer.
This does seem a little far-fetched, and requires justification. The only
other way to avoid this problem is to have stardate issue 21 start in the
middle of 2322, instead of at the beginning of 2323. This would mean that the
change to the new system would occur sometime around SD [20]4930. That is no
neater than SD [20]5006.0, so there would be no reason at the time to use it.
(The use of SD [20]5006.0 is justified only by the fact that it is the
beginning of a year.)
Another suggested explanation is that the Earth's weather modification system
-- mentioned in the TNG epiode "True Q", and hinted at in TVH:STIV -- was used
to change the climate in France deliberately. The ST:TNG episode "Sub Rosa"
supports this hypothesis, by confirming that Federation technology would be
sufficiently powerful to do this. It seems unlikely, though, that the UFP
would want to make such a major change to the climate. This must also be
taken into consideration when evaluating the "Probe" explanation above -- why
wasn't the weather modification system used to repair the climate after the
probe changed it? It is possible that the weather modification system is not
capable of making such major changes.
If you do the calculation, you will find that allowing the 20th issue of
stardates to continue to its natural end before starting the 21st issue would
put "Family" in mid-May. Unfortunately, it would be May of 2394, so that idea
must be discarded.
Intriguingly, a correspondent notes other time evidence in the episode. At
the end of the episode, we see Rene outside, and observe a shooting star
passing the constellation of Orion. Orion is, of course, only visible in the
northern hemisphere during winter, further substantiating the January date.
This seems to support the deliberate weather modification theory.
 
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