Mayonnaise has a sell by date. At what point past that date should you not use it?

If you are already up to the sell by date on unopened mayo, realize that it is very old, perhaps years while it may be safe, it gives me the creeps, see greenstuff's comment about quality. I'd use it in the next month, if you are really desparate not to tos. Once you open it you are on a different clock where food is exposed to oxygen and possible bacteria. Now the sell by date means nothing. You want to use it within a couple of months. Here's a handy site I found mentioned by other members here:
http://stilltasty.com/searchitems/search_page

I agree with Greenstuff on this one. Supermarket mayonnaise is full of stabilizers. Best Foods aka Hellman's can hang out in your fridge for awhile.

Unopened, a long time. Quality will suffer before safety.