How to limit the MDX dataset for the last six months

I have the following MDX query and I'd like to put in another condition that the "[Account Period].[True Prescription Date].[True Prescription Date].ALLMEMBERS" has to be GREATER than OR EQUAL TO last six months. Eg. using today date as 21-Nov-2006 and the dataset should only include the records from 21-May-2006 onward.

SELECT NON EMPTY { [Measures].[Pharmacy DW Count] } ON COLUMNS, NON EMPTY { ([AgencyID].[Agency Id].[Agency Id].ALLMEMBERS * [Account Period].[True Prescription Date].[True Prescription Date].ALLMEMBERS ) } DIMENSION PROPERTIES MEMBER_CAPTION, MEMBER_UNIQUE_NAME ON ROWS FROM ( SELECT ( STRTOSET(@.DrugProtocolCode, CONSTRAINED) ) ON COLUMNS FROM ( SELECT ( STRTOSET(@.DrugDrugName, CONSTRAINED) ) ON COLUMNS FROM [Patient Hospital and Drug])) CELL PROPERTIES VALUE, BACK_COLOR, FORE_COLOR, FORMATTED_VALUE, FORMAT_STRING, FONT_NAME, FONT_SIZE, FONT_FLAGS

Thanks

It depends how you build your True Prescription Date attribute. If we will assume, that it is rebiult daily, and the last attribute member is today's date (i.e. no dates go to the future), then it would be something like

Lag([Account Period].[True Prescription Date].[All True Prescription Dates].LastChild, 182):[Account Period].[True Prescription Date].[All True Prescription Dates].LastChild)

If the rules are more complex, then it is probably best to build named set inside MDX Script which will resolve to the last 6 months worth of dates, and use it instead in the queries.