When I ran this in ssms

SELECT DATEADD(s, DATEDIFF(s, '1970-01-01 00:00:00', 1352325814), '1970-01-01 00:00:00')

I get an Arithmetic Overflow error reported.

And I cant for the life of me figure out why you need the datediff, which means the above can be simplifed to

Select DATEADD(s, 1352325814,'1970-01-01 00:00:00')

which when run returns the number you mentioned, at the top of your post.

In regards to the question what datatype is your original column as i believe it may need to be 8byte integer to hold these numbers, and the output should be a DT_DATE (i believe)