Get the first of the month for a given date in MDX as well as compound interest

How can I get the first non empty day from the calendar dimension.

Actually I am trying to implement a compount interest calculation


I can do that in two ways.  sum of total profit till date of month /principal as of first of date

I am not able to get the principal as of first date. I greatly appreciate is somebod has a code for it. I tried Opening Period but of no success. Getting Nulls at day level. Works at month level

the second way is

CREATE MEMBER CURRENTCUBE.[Measures].CUM_PNL_PCT
 AS ((1 + ([TradeDate].[TDHierarchy].PrevMember,Measures. [CUM_PNL_PCT])) * (1 + ([TradeDate].[TDHierarchy].CurrentMember, [Measures].[PNL Pct By Aum]))) - 1

The calc works if there are only time dimensions (Day on y axis and year and month of x axis).

If I have any other dimension it is failing.  Also I need to reset at any boundary (like new year).

Thanks.