Is this what you are looking for?
DECLARE @agenttime TABLE
(agenttime DATETIME)
INSERT INTO @agenttime SELECT ('2011-09-05 12:12:01.000')
INSERT INTO @agenttime SELECT ('2011-09-06 12:12:01.000')
INSERT INTO @agenttime SELECT ('2011-09-04 12:12:01.000')
SELECT *
FROM @agenttime
WHERE DATEDIFF(DD, agenttime, GETDATE()) >= 30