The stardate is the way date and time are expressed in Star Trek. There is
no canon source on stardate calculation, however, from the hints given in several
episodes, we can draw our conclusions... A typical TNG stardate would be 43185.4
- now, what do these numbers mean?
Some people think the leading four means 24th century. This theory has been
proven wrong in the episode Future Imperfect, in which 5xxxx.x stardates were
used in a period of time well within the 24th century. The second number stands
for the TNG season. It has been established that a season is a year for the
characters, so it is safe to assume that the first two numbers indicate the
year. In "The Neutral Zone", it was established that 41986.0 is in the year
2364. Another thing we know for sure: The number behind the point is the time
of day. In the episode Contagion, some log entries were displayed with stardate
and "normal" time, and the system worked. Summing up, in stardate yyxxx.z, yy
is the year, xxx is the time passed in the year (000 being the beginning, 999
the end), and z is the time of day. However, we do not know if a year still
starts on January 01 - any other date would be possible. We have one clue: In
"The First Duty", stardate 45703.9, earth was shown, and it appeared to be summer
or autumn. Taking January 01 as yy000.z, 45703.9 would be September 14 - well
within the possibilities of what we saw. So, taking January 01 as yy000.z may
not be the accurate stardate, but it will at least come close.
One more thing on stardates: The format is not necessarily yyxxx.z - if there
is a need for accuracy, formats like yyxxx.zz or yyxxx.zzz can be used (and
actually have been used; check the log entries in "Contagion"). Given the appropriate
year, the format would become yyyxxx.z.