Well, as I mentioned before elsewhere, if we were to assume that TOS stardates were introduced prior to "Where No Man Has Gone Before" and after Stardate 9999.9 turned to (1)0000.0 (second log period) you'll get an almost exact and correct difference of 15 years between "Space Seed" and "Wrath of Khan".
Unfortunately, it's appears "Wrath of Khan" also nullifies the concept of different stardates at different locations due to various reasons.
The stardate in Saavik's bridge simulation is basically the same when Starship Reliant
approaches Ceti Alpha V.
But if you ignore the TOS inaccuracies, the TAS stardates, and find a rationalization why there's 18 months of stardate time missing at the beginning of TMP (drydock stay?), there's a strong suggestion that 1.000 stardate digits equal one solar year.
Last but not least, Admiral Kirk's log entries in TMP, too, suggest 1.000 stardate digits for one solar year, if you do the math.