DateTime hiarchy in the cube, MDX

Hi everyone

I am new to MDX and I have a cube that has a datetime dimension and has a hirachy defined with Year, month, day, hour and then 15 minute interval

What I wanted to do is, if a user says give me data from 6/15/2010 till 1/1/2012

writing an mdx to return data based at the leave level will take alot longer to return the data. How can I affectively write an mdx such that it wil return data using 6/15/201 till 7/1/2010 on daily basis and then from 7/1/2010 till 1/1/2011 on monthly basis and then from 1/1/2011 till 1/1/2012 yearly aggregation.

This way less time will be taken and values will be used that were aggregated when teh cube got processed.

Right now my query does something like this, how can I improve it to pull records out at the correct level of time dimension hiarchy. Date Time Id is the last level in teh hiarchy and pulls ALLL teh records one by one.

Select [Measures].[Value Actual SO2 Emissions],
	[Measures].[Max Point Type Reading]
	}on columns,
NonEmpty
NonEmpty([Dim_DateTime].[Date Time Id].&[201011010400]: [Dim_DateTime].[Date Time Id].&[201112010500]*
	[Dim_TimeZoneConvert].[Time Zone Key].[Time Zone Key] *
	[Dim_Item].[Item Id].[Item Id]
	)on Rows

Thanks