What is the expected shelf life of salted butter?
There is a possible case that the butter isn't old. McGee writes that butter made from unpasteurized cream should be eaten within 10 days, else its unique aroma is lost. Maybe the addition of salt and/or other preservatives stretch that to over 20 days, or the manufacturer is being optimistic. 

On the other hand, even if the butter is old, there is no need for concern. Butter doesn't catch germs or molds. It can go rancid, but this is a chemical change, which happens slowly. The affected parts are visible, and after removing them, there is no problem with the remaining butter - this is no invisible penetration as with mold. The butter must get really old (maybe > 6 months) to get small pockets of rancidity on the inside which are too much trouble to remove. And even if they aren't removed properly, eating small amounts of rancid butter isn't a health concern (although the taste is ruined). 

Butter also doesn't undergo signinficant changes when it ages - it isn't like eggs, which are good to eat for weeks, but behave differently when fresh. So eating old butter isn't worse than eating fresh butter (with the exception of the raw-cream butter mentioned above). 

For these reasons, I wouldn't worry about the butter. Unless it was sold as raw cream butter, it is probably old stock. But it is still good to eat, and will be good to eat well past its stamped date. If it should develop an oxidized layer on the outside, just cutting it away will be good enough.