Since it is not more than 6 months past the expiration/use by date, I would use it. At some point, there is some kind of mold that can be in old mixes, like pancake mixes. But I can't remember the details---I'll try to locate it in a minute. But the expiration/sell by date isn't designed for the mold issue. It is determined by how long the baking powder in the mix will still cause the proper rising of the mix. At under 6 months past, I would think it would still be fine. The only problem might be that it rises a touch less than it might otherwise. But it probably won't even do that. After 6 months or more past the date, I would toss it, just because you'd end up with a flat whatever it is.
But, the mold issue is only for people who have ALLERGIES to molds. And even then, apparently, the sell by date has ZERO to do with whether or not the product contains any molds.
Here is the snopes on it: http://www.snopes.com/medical/toxins/pancake.asp