Normally, milk itself takes about two weeks to expire, at least from my experience buying it from our local grocery store.
However, after buying a carton of soymilk, I noticed a shocking thing; it takes more than 2 months for it to fully expire (I bought it at the end of May, it expires on "August 23rd, 2013"). So what's the reason behind this? I'd assume that most soymilk and milk don't use preservatives, which would rule that out, but what could possibly allow soymilk to take longer to expire?
Best Answer
What do the dates mean?
On their website FAQs, the Silk manufacturer describe the date on their package:
If the milk is labelled best by, that is an indicator of quality, not safety. However, normally milk in the US is labelled with a Sell By date, and should be good for a week or so after that date, or for about a week after being opened, whichever comes first.
What are these milks?
Now consider what these two different products are. According to the Go Ask Alice column published by Cornell:
Milk is a complete food for a growing animal (or human). This makes it an ideal growth medium for microfauna of all types, as it contains everything they need—including easily available sugars to jump start their growth.
Soy milk on the other hand, while derived from the seed intended to grow a full plant, is has a very different profile. It is almost devoid of sugars are readily accessible fats, making it a less ideal growth medium for micro-fauna. This is not say they won't grow, but they may not colonize as readily and grow as quickly.
Conclusion
Therefore, you would expect soy milk to have a longer unopened shelf life than milk as it simply is not as ideal a growth medium. Silk implies it may last longer after opening as well, but being conservative they do not promise that.
Milk will have a shorter shelf life before opening (it is not sterile, and will go bad even in the refrigerator), and definitely will last less time after being exposed to the environment.
They are simply two different products, with very different contents; the fact that both are named "milk" is not an indicator.
See also: Expiration Date on Milk