Dates while querying not returning proper output in SQL Server

I have a query when I run like this

select emp_name, dt_of_join 
from emp_mst 
where dt_of_join = '2015-09-14'

I get one record from the table.

But when I try to run in dynamically like below

SELECT emp_name, Dt_Of_Join 
FROM emp_mst 
WHERE Dt_Of_Join =  DATEADD(month, -6, GETDATE())

it doesn't return any records. WHY ???

I am using SQL Server 2008.