WHERE DateCol < DATEADD(dd, DATEDIFF(dd, 0, DATEADD(dd,-30, GetDate())), 0)
AND DateCol > DATEADD(dd, DATEDIFF(dd, 0, DATEADD(dd,-90, GetDate())), 0)
The DATEADD-DATEDIFF
truncates the time part, so that 30 days ago means midnight 30 days ago.