They are essentially the same thing. Sometimes you have a last modified timestamp handy and sometimes you have a revision number handy. I would agree that you

They are essentially the same thing. Sometimes you have a last modified timestamp handy and sometimes you have a revision number handy. I would agree that you shouldn't be doing signifiant calculations -- like checksumming the contents of a file -- to determine the current etag value. Also, the excessive semantics of timestamps is a negative. It adds complication which sometimes trips developers up but doesn't improve the caching.