To subtract days, use a negative number in the second parameter
9> SELECT DATEADD(dd,12,"24 March 1964")
10> GO
-----------------------
1964-04-05 00:00:00.000
(1 rows affected)
The syntax for DATEADD requires 3 parameters.
DATEADD(what_to_add,number_to_add,date_to_add_it_to)
The first parameter can define what you are wishing to add to the date.