if object_id('[tb]') is not null drop table [tb]
go
create table [tb]([begintime] datetime,[endtime] datetime)
insert [tb]
select '2012-05-01','2012-05-30'
go

alter table tb 
  add midtime as dateadd(dd,datediff(dd,begintime,endtime)/2,begintime)
go

select * from tb

/**
begintime               endtime                 midtime
----------------------- ----------------------- -----------------------
2012-05-01 00:00:00.000 2012-05-30 00:00:00.000 2012-05-15 00:00:00.000

(1 行受影响)
**/