Month difference in an org table

months.org.txt

#+BEGIN_SRC emacs-lisp
    (defun months-diff (a b)
      (let* ((date-a (mapcar #'string-to-number (split-string a "-")))
             (date-b (mapcar #'string-to-number (split-string b "-")))
             (months-a (+ (* 12 (car date-a)) (cadr date-a)))
             (months-b (+ (* 12 (car date-a)) (cadr date-b))))
    
        (+
         (- months-a months-b)
         (if (< (caddr date-a) (caddr date-b)) -1 0))))
#+END_SRC

#+RESULTS:
: months-diff

| Foo | 2018-03-01 | 6 |
| Bar | 2018-02-01 | 7 |
#+TBLFM: $3='(months-diff "2017-09-01" $2)