попробуйте
select count(*),DATEADD(DD, DATEDIFF(DD,0,DateColumn), 0)
from SOmeTAble
group by DATEADD(DD, DATEDIFF(DD,0,DateColumn), 0)
пример
select count(*),DATEADD(DD, DATEDIFF(DD,0,crdate ), 0)
from sysobjects
group by DATEADD(DD, DATEDIFF(DD,0,crdate ), 0)