create table #date
(
number int identity(1,1) primary key,
date datetime
)
insert into #date select '2009/1/11 17:00'
insert into #date select '2009/1/12 12:00'
insert into #date select '2009/1/12 10:00'
insert into #date select '2009/1/12 4:00'
insert into #date select '2009/1/11 18:00'
insert into #date select '2009/1/11 12:00'
insert into #date select '2009/1/11 5:00'
insert into #date select '2009/1/11 2:00'

select (d2.number-d1.number) number
from
(
select number,date from #date where date in
(select max(date) from #date group by convert(varchar(10),date,120) )
) d1
,
(
select number,date from #date where date in
(select min(date) from #date group by convert(varchar(10),date,120) )
) d2
where convert(varchar(10),d1.date,120)=convert(varchar(10),d2.date,120)