In the original series, stardates were random nonsense that nobody was really supposed to pay much attention to. The adventures of the Enterprise were apparently presented as pages out of Kirk's log.
I'd say the stardates were turned
into nonsense when the network decided to broadcast the original episodes out of their production order (and Gene Roddenberry felt forced to play along, hence his odd stardate explanation in The Making of Star Trek
Even in production order
there are occasional glitches (it was a television show, not rocket science) but there's definitely a red thread hinting that they intended chronological stardates, IMHO.