The StarDate is a fictious value that was supposedly used to keep track of
mission time while starships were away from known federation planets. Because
of the time distortions encountered by moving at the speed of warp, the
Julian calendar would no longer work, so subspace beacons were needed to
give 'star dates" that account for subspace time fluxes.
But please read the FAQ about the Stardate.
NOTE: the formula gives a "BEFORE WARP" stardate because star dates come do not come
into existance until the year 2323 (when it is rumored that warp technology
was invented, and hence the need for a time system which takes into account
time distortion, subspace anomalies, etc).
It use the following formula:
Take a date such as December 20, 2370 13:12 hrs
YY = 2370 - 2323 = 47
StarDate = 47969.6
The procedure is:
DDD = (334+20)/365*1000 =~ 969
T = (13*60+12)/144 =~ .6
Another formula from Yves - Switzerland
year-2323*1000
+ 1000*(dayOfTheYear-1)/337+lengthOfFebruary
+ (hour*3600+minute*60+second)/86400