You are missing an important point. Even if you introduce your own "new call" that yields slow year and slow month etc, the existing NewGRFs do not know this call and do not use it.
Everything is fine if you only use default vehicles, but that is not enough. This is a big issue with the game development, as there are many many NewGRFs that will either:
not benefit from the new feature
they do not need to know this feature, this feature is supposed to mask and hide
the game need not to realize that there are 2 clocks in the game
and the code make sure that there is 1 clock in the game
the new calls are just there for future use for people that would like to use and exploit the slow day slow month slow year
the whole idea was to get this feature in the game without realizing the game that it was introduced.
when the game ask for a date it will always get the SLOW DATE
there will be no other date to confuse the game.
and since almost everything is tight up with days from the beginning aka _date parameter there will be no problem
since there is 1 counter of days and that is always increasing at a slower rate.
I invite you to look into newgrf.cpp line 5750 or so, function GetGlobalVariable. You see there there are various ways in which NewGRFs access the global variables _date, _cur_year, cur_month etc. Mind you, this is ALL newGRFs not just vehicle ones, e.g. also industries. You can NOT introduce new parameter values or change the meaning of the existing ones without breaking compatibility with older GRFs, therefore, you can only change what the function returns for each parameterer.
i look the code end??
there will be no problem calculating that NO PROBLEM AT ALL
So let's say you replace current year with your "slow year", but leave everything else untouched. This will cause all newgrfs to introduce stuff to the game according to the slow year, which is what you want. But now years have N * 12 months, as far as NewGRFs are concerned, and that might break e.g. NewGRFs that accumulate usage or production or stockpiles on yearly basis. In addition, variable snowline height or production on specific months of the year will be glitchy, e.g. a factor 2 you will experience 8 seasons per game (two springs, two winters etc)
Nope the year do not have 12N months now
there will be N game year inside a slow year all of them with 12 game months.
you do realize that all the call in the game are still done in the normal year?? and not in the slow year??