Okay, before reading this article, you might want to read this tutorial.
http://www.create-games.com/article.asp?id=1908
This tutorial tells how to make a basic real time clock setup, and tells some simple inspiration for different uses. Being a tutorial of course though... It's a little boring Hey it was my first tutorial/article EVER
But I've learned a lot since then. In fact, counters aren't even needed to do this.

Here's a major thing I've learned...
Instead of setting counter equals 3 change animation to 3 and such, you can do somthing much easier and less time consuming.
Say I have this tree that changes every month. Tree seen below.
Image
Okay, see it's kinda a lime green and dark green now. But say this is what I want the tree to look like in June, instead of counter equals 5 change animation to June, you can put all of the animations in one animation. Like this seen below:
Image
Okay, now I organize then in ascending month order, you might have to copy and paste some animations right next to each other though. But put a place holder in front of your January tree for frame 1. When messing with animation frames, frame 1 is considered value 0. So frame 2, is technically January. Okay, now go to the events.
Always:Force animation of Tree to "month of( "Date & Time" )"
("Date & Time" is the date & time object that you should have placed down as talked about in the first tutorial.)
And vala!(or however you spell it.)
Now you can have your tree change with the month without having to make 12 events.

Okay, you can do this with a lot of stuff like setting your lighting effects to hour of date and time. Sorry about not a very good article, I wanted to write more, but I got to go for now...