The 'cert' attribute used by tei:date (among other elements) takes values from the enumeration data.certainty. At http://www.tei-c.org/release/doc/tei-p5-doc/en/html/ref-data.certainty.html that enumeration is documented as containing the values "high", "medium", "low", and "unknown". As far as I can tell, however, nothing in the reference material says what those values mean. For high, medium, and low it seems to be passably obvious, but does cert="uinknown" mean
(a) Lower than "low": the certainty of this value is nil; the value is not known at all.
(b) Not saying: the certainty of this value may be high, or medium, or low; we don't know how certain it is.
Both (a) and (b) seem to me to be things one might wish to say, in different circumstances, so neither seems a priori impossible as an interpretation. I think the Guidelines should say what the value means.