Problem with date calculations in a table

[Nick Dokos <nicholas.dokos@hp.com>]

I've been trying to make a table with the following structure: column 1
contains either a date or is empty; column 2 contains a date that's
calculated from the corresponding date in column 1 (if column 1 contains
a date) or is empty (if column 1 is empty). In the example below, column
2 is supposed to be the date 12 hours or 0.5 days after the date in
column 1 (assuming that it is non-empty).

I have tried various formulas as shown in the tables below where row 1
contains a date in column 1 and row 2 is supposed to be empty:


| start                  | ETA (start + 12 hrs)       |
|------------------------+----------------------------|
| <2008-08-14 Thu 18:15> | <2008-08-15 Fri 06:15>     |
|                        | <+1-01-01 Sat> ? 0 + 0.5 : |
#+TBLFM: $2=(date($1)?$1+0.5:string(""))


| start                  | ETA (start + 12 hrs)                                    |
|------------------------+---------------------------------------------------------|
| <2008-08-14 Thu 18:15> | <2008-08-14 Thu 06:15> ? <2008-08-14 Thu 06:15> + 0.5 : |
|                        |                                                         |
#+TBLFM: $2=($1?$1+0.5:string(""))


| start                  | ETA (start + 12 hrs)                                         |
|------------------------+--------------------------------------------------------------|
| <2008-08-14 Thu 18:15> | 733269.76 != <+1-01-01 Sat> ? <2008-08-14 Thu 06:15> + 0.5 : |
|                        |                                                              |
#+TBLFM: $2=(date($1)!=date(0)?$1+0.5:string(""))


| start                  | ETA (start + 12 hrs)            |
|------------------------+---------------------------------|
| <2008-08-14 Thu 18:15> | <2008-08-15 Fri 06:15>          |
|                        | <+1-01-01 Sat> != 0 ? 0 + 0.5 : |
#+TBLFM: $2=(date($1)!=date(<+1-01-01 Sat>)?$1+0.5:string(""))



but as you can see, there is no formula that gets both rows right. This
is a subset of all the trials that I have done: none seems to work. I'm
almost certain that the error (if it is indeed an error and not my own
stupidity) lies with calc-eval, not with org-table, but I am just not
ready to wade into those waters yet.

If somebody has a different formula, a work-around for any of the above
or some related formula, or can give me the proper head-whack to point
me in the right direction, I'd appreciate it very much.

BTW, I tried the if(a,b,c) form of the conditional as well, but it does
not make any difference afaict: they seem to be entirely
equivalent. That's what I would have expected, but desperate situations
call for desperate measures!-)

Thanks,
Nick

Version info:
Linux 2.6.24-19-generic #1 SMP Fri Jul 11 23:41:49 UTC 2008 i686 GNU/Linux
GNU Emacs 23.0.60.1 (i686-pc-linux-gnu, GTK+ Version 2.12.9) of 2008-07-01
Org-mode version 6.06b