There has  been a  long discussion  on this subject  some time  ago (one
year?) on several lists. The conclusion was:
- the  *complete* list  of *all*  admissible values  is to  be found  in
  RFC822;
- if only three characters are  available (like in CP's timezone field),
  then it may  be *tolerated* to use only the  three first characters of
  those RFC822 timezone values normally needing five characters.
                                                       /AF