I’m not sure what you mean - I was talking about making the game match the real-life calendar, beginning as of the game’s alleged start year. As an optional mode; instead of days-per-season with years which are oddly only two months long, you get a game-year passing in the course of 365.25 days. Handling leap years is fairly simple, too, for extra grognardianness:
if (year is not divisible) by 4) then (it is a common year)
else if (year is not divisible by 100) then (it is a leap year)
else if (year is not divisible by 400) then (it is a common year)
else (it is a leap year)
(And I would by duly impressed if someone played long enough to see the 2400 Leap Year. But modifying the start date seems more likely to result in seeing a leap century.)
The game already handles weather variations - snow in early Spring and late Autumn, etc - so whilst I’m not sure how it works internally, I think it should be very possible to weight the weathers towards the height of the calculated calendar seasons, so that it gets to be wintry in wintertime and hot in summer, etc.