How to transform abbrevated dates in org table

Question

The input column represents the year without 201 and month and day without leading zeros. I must transform these into full date strings.

After a couple of hours to take step 1, I'm wondering if there is a more elegant way to achieve the steps left:

1. Year: Find first first digit from left: Append 201 before (done)
2. Month: Find 5th digit from 1 to 9: Append a 0 before
3. Day: Find last digit from right which is between 1 to 9: Append a 0 after
4. Insert hyphen after year, month and date

|-------+----------+------------|
| input |   output |   expected |
|-------+----------+------------|
|  9725 |  2019725 | 2019-07-25 |
|   811 |   201811 | 2018-01-01 |
| 71212 | 20171212 | 2017-12-12 |
|-------+----------+------------|
#+TBLFM: $2='(baz @0$1)

(defun baz (cell)
  (concat "201" cell))

Explanation: The input represents the batch processing of approx. thousand grocery receipts. The goal was to hit as few key strokes as possible.

Answer 1

If you don't mind using the date command, you can try the code below which I think addresses the four points you describe. It works fine with your example, however AFAICT it's impossible to distinguish between 11-1 and 1-11, so here I'm assuming the month is always two digits when the date is 7 digits long.

(defun fix-date-string (date)
  (let ((yy (substring date 0 4))
        (mm (substring date 4 5))
        (dd (substring date 5 6))
        (mmdd (substring date 4)))
    (cond ((= (length date) 6)
           (concat yy "0" mm "0" dd))
          ((= (length date) 7)
           (concat yy "0" mmdd))
          (t date))))

(defun baz (date)
  (let* ((it (concat "201" date))
         (it (fix-date-string it))
         (it (format "date -d '%s'" it))
         (it (shell-command-to-string it))
         (it (string-trim it))
         (it (date-to-time it))
         (it (format-time-string "%F" it)))
    it))