Default Value Of Table
A computed column would solve the issue and probably be better than having triggers:

CREATE TABLE #Moofasa
(
  gotitdate DATE,
  dunitdate DATE,
  delayed AS COALESCE( DATEDIFF(day, gotitdate, dunitdate), 0)
) ;


Tested at: rextester.com:

INSERT INTO #Moofasa 
  (gotitdate, dunitdate) 
VALUES
  ('2016-07-01', '2016-07-05'),
  ('2016-07-01', '2016-07-04'),
  ('2016-07-04', '2016-07-08'),
  (NULL,         '2016-07-09'),
  ('2016-07-09', NULL),
  (NULL,         NULL)  ;

SELECT * 
FROM #Moofasa ;


gives:

gotitdate   dunitdate   delayed
01.07.2016  05.07.2016  4 
01.07.2016  04.07.2016  3 
04.07.2016  08.07.2016  4 
NULL        09.07.2016  0
01.07.2016  NULL        0
NULL        NULL        0