->?
declare @dt datetime
set @dt='2012-08-21'
select qfrq between convert(varchar(7),dateadd(month,-1,@dt)),120)+'-25' and @dt from tab

------解决方案--------------------
SQL code

declare @d0 datetime
set @d0='1900-1-25'
declare @time datetime
set @time='2011-1-1'
--用偏移量计算
select case when DAY(@time)>=25 then DATEADD(MONTH,DATEDIFF(month,@d0,@time),@d0) else DATEADD(MONTH,DATEDIFF(month,@d0,@time)-1,@d0) end

------解决方案--------------------
SQL code
/*创建Nums表，1W行                                   
 */ 
CREATE TABLE dbo.Nums(n INT NOT NULL PRIMARY KEY);
DECLARE @max AS INT, @rc AS INT;
SET @max = 10000;
SET @rc = 1;

INSERT INTO Nums VALUES(1);
WHILE @rc * 2 <= @max
BEGIN
  INSERT INTO dbo.Nums SELECT n + @rc FROM dbo.Nums;
  SET @rc = @rc * 2;
END

INSERT INTO dbo.Nums 
  SELECT n + @rc FROM dbo.Nums WHERE n + @rc <= @max;
GO

/*函数用于返回时间的月份序列，类似方法可以产生日序列，周序列 等等
        参数：@s 开始时间
                    @e 结束时间                    
 *磊仔 2012年5月29日创建                                      
 */   
Create function dbo.fn_DayList(@s as datetime,@e as datetime) returns table
as
return
with CET1 AS
(        SELECT  cast(convert(varchar(6),dateadd(mm, n - 1,@s) ,112) as int) as [Month],--月序列
            dateadd(dd,25,dateadd(mm,-1,dateadd(d,datediff(d,0,dateadd(mm, n - 1,@s)),0) - DATEPART(dd, dateadd(mm, n - 1,@s))+1))as StartT,
                dateadd(dd,25,dateadd(d,datediff(d,0,dateadd(mm, n - 1,@s)),0) - DATEPART(dd, dateadd(mm, n - 1,@s))+1) as EndT
        FROM dbo.Nums
        WHERE n <= DATEDIFF(mm, @s, @e)
)SELECT * FROM CET1        
GO

--创建完以上表值函数后就可轻松调用，且性能极佳

declare @time datetime
set @time='2011-7-25'
select StartT, @Time as NowTime 
    from dbo.fn_DayList('19000101','20500101') 
        where StartT <= @Time and EndT > @Time