The encyclopaedia usually has its date prominently displayed on the cover because it's part of the business model: Nudging people into upgrading last year's edition because it's "hopelessly out of date".
Well, pre-internet our family only ever had 1 set and they retain a very high base value even as they age, never felt a need to update to a newer set continually.
Point still stands though, are PG's essays devalued because he puts a year date on them? I don't think so.
