1000 star dates equal a year roughly... Is this not defined somewhere?
Since it holds together so amazingly well for TOS, it may be the people slapping stardates onto those episodes did indeed think in these terms. Or at least some of the people did.
Basically, though, 1000 SD/season was decided upon for the first season of TNG, and they stuck to it thereafter - the big question is, does a season equal a year? In most writing, it does, but sometimes the writers make mistakes there, and sometimes it's more convenient to claim that five years have passed rather than the actual three seasons if it allows an older child actor to be used for Molly O'Brien...