This article is from the Stardates in Star Trek FAQ, by Andrew Main zefram@dcs.warwick.ac.uk with numerous contributions by others.
It has been calculated that stardates begin on 2162-01-04. It is quite
possible, of course, to continue stardates back in time from that point, using
negative issue numbers. Within each issue the stardates will still increase
in the direction of advancing time; only the issue numbers will be unusual.
To start with, midnight on 2162-01-04 is stardate [0]0000. This means that
midnight on 2162-01-03 can be referred to as stardate [-1]9995. SD [-1]0000,
consequently, was 2156-07-14. The fact that stardates were not actually in
official use at that time is irrelevant.
It seems possible that negative-issue stardates are used in the 23rd century
to refer to events prior to 2162. When going as far back as the 20th century,
though, the old-style Gregorian calendar has always been used by the crew of
the Enterprise. In fact, they have never been canonically observed to use
stardates to refer to any time earlier than 2260. Of particular interest is
the final log entry in ST:FC: "Captain's log, April 5th, 2063...", indicating
that in official contexts stardates are not used for dates that early, or at
least are not universally used.
It is possible to continue stardates back to the 20th century, but the issue
numbers get sufficiently large (in the negative direction) to be awkward.
This table shows the issue origins for the next few years:
[-36]0000 1964-11-18 [-24]0000 2030-08-04 [-12]0000 2096-04-19 [-35]0000 1970-05-11 [-23]0000 2036-01-25 [-11]0000 2101-10-11 [-34]0000 1975-11-01 [-22]0000 2041-07-17 [-10]0000 2107-04-03 [-33]0000 1981-04-23 [-21]0000 2047-01-07 [-9]0000 2112-09-23 [-32]0000 1986-10-14 [-20]0000 2052-06-29 [-8]0000 2118-03-16 [-31]0000 1992-04-05 [-19]0000 2057-12-20 [-7]0000 2123-09-06 [-30]0000 1997-09-26 [-18]0000 2063-06-12 [-6]0000 2129-02-26 [-29]0000 2003-03-19 [-17]0000 2068-12-02 [-5]0000 2134-08-19 [-28]0000 2008-09-08 [-16]0000 2074-05-25 [-4]0000 2140-02-09 [-27]0000 2014-03-01 [-15]0000 2079-11-15 [-3]0000 2145-08-01 [-26]0000 2019-08-22 [-14]0000 2085-05-07 [-2]0000 2151-01-22 [-25]0000 2025-02-11 [-13]0000 2090-10-28 [-1]0000 2156-07-14
 
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