How to get period date of for a given month from a given date in mdx for SSRS report (mm/dd/yyyy)

I have a situation,  where i need to write expression Period to date(PTD). i want to know how to get the period date. i want you to help in writing Period date or else is there any function to get period date for a given date(the  date is given from the parameter dynamically) in MDX for SSRS report

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ram