I wouldn't. Grain type products have weevil eggs that will hatch inside the packaging after a period of time. I wouldn't want to take the chance of having bugs in my cake.
If you want to read about these disgusting things...
http://www.pestproducts.com/weevils.htm
Bugs can be a problem. The first thing is to open the pkg. and give it a good shake, if nothing falls out, no bugs have evaded from the outside. Next is the weavel problem, and it is real. Open the bag and look, there can be webbing and live animals, or dead. They're not hard to spot. Ok, if you're past the bugs and all seems ok, you can cook it. But, the flavor may well have deteriorated. For the price of a box of cake, maybe the best is to heave it and buy anew.
Here, in a very hot climate, I use the mixes within 3 months of buying them or I put them in the fridge. I'm also very careful when buying the boxes to check the out date. The US loves to sell the outdated product (who can blame them) to the uneducated buyer and then of course, the consumer dumbs, dumbs along and buys them. Bugs! Surprise!
I wouldn't use it. But, I'm a freak about that kind of stuff. Anything with expiration on it, if it's expired out it goes.
Wow you guys! I remember these bugs from when I was a kid! We had them in our cracked wheat, my mom would say just scrap them out and eat it anyway! 
Good thing they dont make you sick! Sorry I just went down memory lane for a sec. ok I'm so glad I know what they are now, that was bugging me
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