Thanks Bastien, it's what I am expecting :)

> From: bzg@gnu.org
> To: truonghatsts@outlook.com
> CC: emacs-orgmode@gnu.org
> Subject: Re: Get the maximum date within a list of dates
> Date: Sun, 22 Dec 2013 08:27:08 +0100
> 
> Bastien <bzg@gnu.org> writes:
> 
> > Put your cursor in @2$2 then C-c ^ T to sort by "reversed" date
> > (oldest last).
> 
> Also remember you can use ELisp in formulas:
> 
> | Date 1                  | Date 2                  | Duration |
> |-------------------------+-------------------------+----------|
> | [2013-12-21 sam. 00:00] | [2013-12-22 dim. 00:00] |  86400.0 |
> #+TBLFM: $3='(- (org-time-string-to-seconds "$2") (org-time-string-to-seconds "$1"));L
> 
> PS: note the quotes around "$2" and "$1", they are necessary so
> that the date is not literally inserted in the formulas when Org
> is computing it.
> 
> HTH,
> 
> -- 
>  Bastien