Unofficial, sure, but it's also the sole accepted calendar and has never, as far as I know, been contested by anyone.
And just because it starts with Dwilight's opening doesn't mean it's usefulness is limited to Dwilight. There's absolutely no reason it can't be exported to all continents. Our calendar has negative years, after all. And if that isn't wanted, then the same calendar could be used, but with a different name, with year zero being BM's opening instead.
It's not because the game never had an official calendar that it can't ever have one.
The ways in which a year is defined in the game is variable, though. There's the passage of time (seasons, normal aging) and then "accelerated" years; wounds are the only thing coming to mind right now, but I think there's a few other ways in which a year isn't always a year.
