Shelf Life of Cake Mixes

2 teaspoons of baking powder is what I add to expired cake mixes. I have had good luck with the three major brands 2-3 years (I sometime freeze them for a year) after their expiration date.
I also put cornmeal and flour and baking mixes in the freezer for storage.

I have 25-30 year old letters from at least three national companies saying canned goods are safe to eat as much as 5 years after purchase. Even after their expiration date.Nutritional value and color may have changed, but still safe to eat. GG Vi

The expiration date is actually the 'sell by' date and almost all packaged foods stand up well for a long time after that date. Even canned foods are good for quite a few years after and simply lose some flavor and quality. I've used cake mixes and brownie mixes for up to two years after the date with no problem. I personally think it depends on how the foods are stored. Cool, dark storage definitely helps preserve them longer. OliveOyl's suggestion of adding a bit of baking powder when in doubt is excellent advice.

The reason for the expiration dates are for the levening agents in the cake. I would feel comfortable going six months out.

Please do not throw the cake mixes away if they are expired. They are still edible and many times i mash up bananas or other fruit and put in them before baking. In working with numerous food banks & pantries, I've found that food is good for five years past the expiration date. That is what they teach you. Too many people are willing to eat a possibly "fallen" cake. If you can't do the thought of expired items, please, drop them off at your local food pantry.

I have used expired cake mixes (no more than 1 year past expiration) and they do not rise well. It appears the baking powder becomes inactive, so if you want to use an old mix, add a teaspoon or so of baking powder. If the mix is more than a year old I would throw it away.