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 second season ST:DS9 episode "Second Sight" (stardate
47329.4), Sisko states that it is the fourth anniversary of his wife's death.
From "Emissary" it is known that Jennifer Sisko died in the battle of Wolf
359. That was sometime between the starts of the fourth season ST:TNG
episodes "The Best of Both Worlds, Part II" (SD 44001.4) and "Family" (SD
44012.3). This dating information makes four years something between 3328 and
3339 units, and one year something in the range 832 to 834.75 units. This is
incompatible with the notion that 1000 units is a year.
The real reason is (probably) that the writers made a mistake. An isolated
case like this might normally be ignored, but in this case there is an
explanation.
It has been established that Deep Space Nine works to Bajoran time, with 26
hours per day. It seems logical to extend this to years, in which case Sisko
may have been referring to Bajoran, not Terran, years. This explanation has
the helpful consequence that we now know a Bajoran year to be about 304 Terran
days long. One might also conclude that a Bajoran year is also (usually) 281
Bajoran days long, but that conclusion relies upon Bajoran hours being the
same length as Terran hours.
 
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