I posted a year ago about a printed source I found stating that, as I had long suspected, stardates were based on Julian Dates. I was reviewing the Memory Alpha page on stardates and came across some information on the subject, which had originally been posted on Star Trek Fact Check. Apparently, the use of the Julian Day system was suggested by Kellam de Forest (not to be confused with DeForest Kelley) who worked as a script researcher. Here are some notes he made on the (second) pilot:
(Page 2, Scene 3)But on star date 1312.4 – Astronomers already have adopted a method of dating which makes possible the counting of the number of days elapsed between widely separated observations called 'the Julian Day'. Today July 14, 1965 is 2,438,956 in Julian days. A Julian cycle is 7,980 years, and the Julian day measurement would be scientifically authentic. Suggest “on Julian B 1312.4”. This date would be August 5, 3271.(Page 65, Scene 175)C-1277.1 to 1313.7 – We presume dates are in days, Kirk would only be 36 days old. For conventional dating suggest 3235 to 3271. For Kirk’s birth date in Julian system figure would be in millions. If desired, can be calculated.
He seems to be assuming that the show was set a thousand years in the future of when it was later established to occur. Kirk is now considered to have been born in 2233, and the five-year mission took place in the late 2260s. Since the Julian Period lasts 7980 years and the current one began in 4713 BCE, the next Julian Period begins on January 1 of the year 3268. Thus, a Julian Date of 1312.4 would be 1,312 days, or about three-and-a-half years, later, on August 5, 3271, as he says. Presumably "Julian B" refers to the next Julian Period, so the current one is Julian A.
It has long been a mystery what exactly C-1277.1 means on Kirk's tombstone. Obviously, Kirk is more than 36 days old; more like 36 years. Some have suggested that C stands for "captain", and that these are the dates of his command. But perhaps it's for Julian C, indicating that the first stardate is in a different period than the second, although it's hard to infer the specifics. The year 3235 lies between Julian days 2,902,620 and 2,902,984. But it appears that somebody else picked the stardates, and de Forest was trying to retcon them as Julian Dates.
So we now have a genealogy going back to the French Revolution, starting with the Republican Calendar and decimal time, which passed into astronomy through Laplace, being combined with Herschel's Julian days, eventually to inspire the creation of stardates.