Why not? Paradise City should have season changes. Or there are no winter, spring, etc?
Some ideas about this:
1. Seasons change every two in-game days
2. There are no apples in winter, or they give less food points to colony, like 0,5 for every apple, less zombies, but they become kinda stronger because there should be balance after all. Like ice zombies? Food prices are little higher; may be some new items available only in winter (every season has special items; these are sold in special season); though, in winter it snows, so new snow barricades would make game little easier.
3. Spring - apples give 1 point, as usual. Zombies are normal. Prices dont change. Maybe some items from colony (kinda help things - upgrades for SWATs, or new stronger types of SWATs. They last for only 8 in-game days, but this should make winter less difficult to survive)
4. Summer! Apples give 1,5 points, may be a new sim stat 'heat' (dont remember who suggested this, but its a good idea). All other seasons have this sim stat to, but then they are called 'illness'
5. Autumn. Apples give 2 points, zombies are little stronger, there are MANY of them, colonists die every day because of illnesses.
6. Some changes for SWATs every seasons that make balance, like some SWATs in summer have less HP and cant throw grenades; this should make it little harder, and makes balance.
Seasons should have new music (maybe) and LITTLE graphical changes (if you want, but HQ should be covered with snow at winter, because everybody like playing snowballs with grenades! :) )
And please think about this suggestion a little more, because it SHOULD make gameplay little better and more interesting. And, I hope, it WILL.