Conditions in Calculated Measures and in KPI

Hi,

I have one Dimension Called CaseCreationDate and I have a measure [total distinct case header count]. I would like to create a calculated Measure called historical cases.

If CaseCreationDate > [2011-01-11] then [total distinct case header count] will be historical cases else new cases

How can I do this in calculated measure using MDX?

Thanks in Advance

MH