datetime, . , (, MAX) ordewr :
select ArchiveDate from (
select DateName(Month,a.createddate) + ' ' + DateName(Year,a.createddate) as ArchiveDate, MAX(createddate) as createddate
from (select CONVERT(datetime,createddate) as createddate from (select '20100101' as createddate union all select '20100201' union all select '20100301') t) a
group by DateName(Month,a.createddate) + ' ' + DateName(Year,a.createddate) ) g
order by g.createddate desc