This article is from the Stardates in Star Trek FAQ, by Andrew Main firstname.lastname@example.org with numerous contributions by others.
From the ST:TOS stardates we know that SD 1530 is 2266-11-21. Extending this
back, SD 0000 is 2266-01-19, well after the beginning of the five-year
mission. To go further back, we must go into an earlier issue of stardates.
In this earlier issue, SD 9995 must be 2266-01-18 (one day before the ST:TOS
stardate 0000). This makes SD 0000 in this issue 2260-07-29. Continuing this
process backward, it should be possible to find a sensible starting point for
The theory is here supported by the ST:DS9 episode "Equilibrium", which puts
Joran Bella's birthdate at stardate 0024.7, and notes that this is in 2260.
(This theory makes it 2260-08-02.) However, the same episode puts his death
as stardate 8615.2 and 2286. This is not quite consistent with this theory,
being two years out.
None of the major real-life space events is a 0000. The 43rd issue before the
classic series has its 0000 on 2030-08-04, and 2030 is supposed to be the year
of Zefram Cochrane's birth; this is a rather implausible candidate for the
origin. The 37th issue before the classic series has its 0000 on 2063-06-12,
just a couple of months after Cochrane's first demonstration of warp drive
(2063-04-05, according to ST:FC). I think the powers that be here missed a
great opportunity here. It looks like stardates just aren't based on any
significant event in space travel.
The 19th issue before the ST:TOS stardates has its SD 0000 on 2162-01-04,
which is tantalisingly close to 2161 (Federation incorporation). Taking this
as the origin of stardates, it could mean that Starfleet originally used
old-style Terran dates, but found them inappropriate for deep-space use. A
few months after incorporation, then, they started up stardates.
If we call these first stardates `zeroth-issue', which will be written like
0000, ST:TOS uses 19th-issue stardates (e.g., 1530 is 2266-11-21). The
partial issue to link the TCFS stardates with the ST:TNG stardates is the 20th
issue, and ST:TNG stardates are 21st issue. This notation provides a
convenient way to refer to stardates a long way from the current time.