sql server 日期操作相关

SELECT datediff( month, '1987/05/02', '1995/11/15' )  -- 102
  SELECT datediff( month, '2013-10-03 22:12:22.000', '1995/11/15' ) --  -215
  SELECT datediff( month, '2013-9-03 22:12:22.000', GETDATE() )  --2   GETDATE()==11-27-2013
  SELECT datediff( month, '2013-10-1 22:12:22.000', GETDATE() )  --1
  SELECT datediff( DAY, '2013-10-1 22:12:22.000', GETDATE() ) --57
  SELECT datediff( DAY, '2013-9-13 22:12:22.000', GETDATE() ) --75
  SELECT datediff( DAY, '2012-9-13 22:12:22.000', GETDATE() ) --440

 select top 1000 * 
--delete 
from  [EDW_PreSource].[dbo].[O******_ENCOUNTERS]
where datediff(DAY, DW_INSERT_DATE, GETDATE()) >= 2 * 30     -- 2 months

 select top 1000 * 
 --delete 
from  [EDW_PreSource].[dbo].[O*****_RETURN]
where datediff(DAY, DW_INSERT_DATE, GETDATE()) >= 25 * 30    -- 25 months