don't put sumary data ( mtd,ytd...agregates) to your fact table. Just make sure that your current time data is correct and OLAP engine will take care of the rest.
For calculating cumulative values use something like this for example:
measure: POS Sales
Time dimension: Date
Sum( MTD([Date].currentMember), [Measures].[POS Sales] )
Sum( YTD([Date].currentMember), [Measures].[POS Sales] )
Sum( YTD(), [Measures].[MTD_for_last_year] )
This is just an example, so don't expect that will work in your case!
I very strongly recommend you to buy OLAP client Knosys-Proclarity, whic will generate simple MDX queries and CA mambers for you, and you can modify them later manually.