DateDiff Returning Dates and Not Integer Values
It's converting the integer to a datetime- the column is a datetime. 

You're seeing the datetime conversion of the results of the DATEDIFF function. Implicit conversion from integer to datetime is in days beginning with 1/1/1900 as 0.

  SELECT CAST(1 AS DATETIME)


Returns

1900-01-02 00:00:00.000


If you want to store the integer result in the column, just change the data type to integer.