date calculation in org table

[The Dude <rafal.kowalski@mac.com>]

Hi all,

I'm having hard time to find a solution to my problem.  I've got a
simple table with a dates column where I set an $sdate variable in the
first row.  What I want to achieve is that the dates in the column 4 are
the sum of the date from the row above plus the number of days in column
3 in the same row apart from the field @4$4 which uses the $sdate
variable; thus my naïve column and field formulae:
@4$4=$sdate + $3::$4=@-1$4+$3

--8<---------------cut here---------------start------------->8---
* Test date table

|   | WP | Days | Arrival Date     |
|---+----+------+------------------|
| # |    |      | <2013-06-05 Wed> |
| ^ |    |      | sdate            |
|---+----+------+------------------|
|   |  1 |    4 | <2013-06-09 Sun> |
|   |  - |    2 | <2013-06-11 Tue> |
|   |  2 |    4 | <2013-06-15 Sat> |
|   |  - |    2 | <2013-06-17 Mon> |
|   |  3 |    1 | <2013-06-18 Tue> |
|   |  4 |    2 | <2013-06-20 Thu> |
|   |  5 |    1 | <2013-06-21 Fri> |
|   |  - |    2 | <2013-06-23 Sun> |
|   |  6 |    1 | <2013-06-24 Mon> |
|   |  7 |    2 | <2013-06-26 Wed> |
|   |  - |    1 | <2013-06-27 Thu> |
|   |  8 |    1 | <2013-06-28 Fri> |
|   |  9 |    1 | <2013-06-29 Sat> |
|   |  - |    3 | <2013-07-02 Tue> |
|   | 10 |    3 | <2013-07-05 Fri> |
|   |  - |    1 | <2013-07-06 Sat> |
|   | 11 |    2 | <2013-07-08 Mon> |
|---+----+------+------------------|
|   |    |   33 | <2013-07-08 Mon> |
#+TBLFM: @4$4=$sdate + $3::$4=@-1$4+$3::@21$3=vsum(@II..@III)::@21$4=@III-1::$sdate=<2013-06-05 Wed>
--8<---------------cut here---------------end--------------->8---

The problem is that when I change the $sdate and recalculate the whole
table with `C-u C-u C-c C-c' only the element @4$4 is set.  It seems
like the column formula doesn't apply to the other fields.

So the question is what am I doing wrongly?  Didn't I get how the column
formula works or is it a problem with the date calculation?

Any help would be very much appreciated,
cheers
-- 
-dude

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