select [Rounded Time] =
dateadd(mi,datediff(mi,0,dateadd(ss,30,a.DT)),0)
from
(
select dt = convert(datetime,'8:24:29.997')
union all
select dt = convert(datetime,'8:24:30.000')
) a
Результаты:
Rounded Time
------------------------------------------------------
1900-01-01 08:24:00.000
1900-01-01 08:25:00.000
(2 row(s) affected)