select
ValueSum = sum(Value),
[DateHour] = dateadd(minute, 15, dateadd(Hour, datediff(Hour, 0, DateQuarter),0))
from
MyTable
group by
dateadd(minute, 15, dateadd(Hour, datediff(Hour, 0, DateQuarter),0))
order by
dateadd(minute, 15, dateadd(Hour, datediff(Hour, 0, DateQuarter),0))