[O] Conditional summing in column-mode?

[Stig Brautaset]

I have the following column-mode defined in =~/org/Holidays.org=:

#+BEGIN_SRC org
  ,#+COLUMNS: %TIMESTAMP(When) %ITEM(What) %CONFIRMED(Confirmed?){X/} 
%DAYS(Days){+}
  ,#+Confirmed_ALL: "[ ]" "[X]"
  ,#+TODO: TODO | DONE CANCELLED
#+END_SRC

Is it possible to have the =%DAYS(Days){+}= part only sum up rows that
have an =[X]= in their =Confirmed?= property?

Stig

-- 
; GNU Emacs 26.0.50, Org mode version 9.0.9