#+name: workingDays
#+BEGIN_SRC emacs-lisp :var hms="0"
  (require 'org-timer)
  (/ (org-timer-hms-to-secs hms) (* 60 60 8))
#+END_SRC

Note the variable reference is prefixed with a "$" to ensure that it
is interpreted as a string.  See the `sbe' documentation for full
`sbe' usage information.

| Days |     H:M:S |
|------+-----------|
|   29 | 236:30:00 |
#+TBLFM: @2$1='(sbe workingDays (hms $@2$2))