How long after the "sell by" or "use by" date is it advisable to eat peanut butter?

For best quality, keep unopened peanut butter no longer than 9-12 months, regardless of the "sell by" or "use by" date. (Mark the date you purchased it on the jar.) Even after it’s opened, peanut butter may still be safely stored on the pantry shelf for awhile. AlgerWiki’s “Expiration Dates” site says an opened jar of processed peanut butter can be kept for 6 months, but it should be refrigerated after 3 months. AlgerWiki says natural peanut can be kept for 9 months (probably referring to an unopened jar).
 
Source(s):
University of Georgia "Peanut Butter: Check It Out!"

AlgerWiki “Expiration Dates”