Hi, 
My report has a Startdate prompt which lets you select 'M' - last month or you can enter any  date as a parameter.
based on what is selected in the prompt I need to display the last month.
If the report is run today choosing 'M' it should show Oct date.
if you enter a value in the prompt like 9/10/2008 then it should show Aug date.
I have to write a formula to get it.

If {?StartDate} = 'M' then
MonthName(month(DateAdd ('M', -1,CurrentDate)),True) & "-" & cstr(DateAdd ('M', -1,CurrentDate),"yyyy")
else
 {?StartDate} ( here I don;t know how to manipulate  {?StartDate} to make it point to last point last month, in this case Aug 2008)