DateDiff Always Return 0

I just discover the result for my query that had use the function dateDiff seems to be in-correct, no matter what's the date, the dateDiff always return a zero. Am I using it in-correctly?

select DateDiff(mm,11/1/2004, 12/31/2005)

SELECT component_id, component_description,
SUM(CASE DateDiff(mm,date_complete,'12/31/2004')WHEN 2 THEN component_qty ELSE 0 END) AS mm1,
SUM(CASE DateDiff(mm,date_complete,'12/31/2004')WHEN 1 THEN component_qty ELSE 0 END) AS mm2,
SUM(CASE DateDiff(mm,date_complete,'12/31/2004')WHEN 0 THEN component_qty ELSE 0 END) AS mm3,
sum(component_qty) as total
FROM view_jobcomponent
WHERE date_complete between '10/1/2004' and '12/31/2004'
GROUP BY component_id, component_description
order by component_id, component_description