insert into a select datetime+3,value  
from a  
where datetime between '2011-11-25' and '2011-11-27'

------解决方案--------------------

没看懂楼主是什么意思,可参考如下:
SQL code
--获取两个时间之内的所有日期

--sql 2000
declare @sdate datetime
declare @edate datetime
set @sdate = '2011-11-28'
set @edate = '2011-11-30'


select 
    dateadd(dd,num,@sdate) dt
from 
    (select isnull((select count(1) from sysobjects where id<t.id),0) as num from sysobjects t) a
where
    dateadd(dd,num,@sdate)<=@edate

/*
dt                                                     
------------------------------------------------------ 
2011-11-28 00:00:00.000
2011-11-29 00:00:00.000
2011-11-30 00:00:00.000

（所影响的行数为 3 行）
*/

------解决方案--------------------
探讨


SQL code
insert into a select datetime+3,value  
from a  
where datetime between '2011-11-25' and '2011-11-27'