King Daniel wrote:
The first mention of "Stardate" in-universe was in the Enterprise episode "Damage". I quite like the idea that the Federation adopted the timekeeping system of their former enemy, the Xindi.
Or it might be more accurate to say that Earth adopted an already existing "stardate" standard that the Vulcans and Xindi used. T'Pol knew what a stardate was when she looked at it, IIRC.