SELECT
  DATEDIFF(YEAR,'20120101','20131231'),
  DATEDIFF(YEAR,'20121231','20130101');

DECLARE @d DATE = SYSDATETIME;

SELECT <cols> FROM dbo.tablename
  WHERE VisitDate < DATEADD(DAY, 1, DATEADD(MONTH, -5, @d));

-- the above evaluates to < '2013-09-04'

DECLARE @Today      DATE,
        @MonthsBack INTEGER;
    SET @Today = GETDATE();
    SET @MonthsBack = 5;

SELECT  1
FROM    dbo.[foo]
WHERE   VisitDate <= DATEADD( MONTH, -@MonthsBack, @Today );

SELECT DATEDIFF(MONTH, '20140130', GETDATE())