This formula is working, however, in my pivot table, I am using dimensions that have different effective dates and the formula seems to be locking into one date for all data and not reacting based on the dimension's effectiev date.
Some things I'd try to debug your calculation would be to duplicate your pivot table and add your date parameters in as measures and see what they are returning.
The Monthstart() function has it's own date shifting parameter so you can take out the addmonths().
Add as measures
date(MonthStart(Min([Effect Date]), -6))
Regards
Anthony
and
date(MonthStart(Min([Effect Date]), -1))
and see if there are different dates appearing in the rows of the pivot table.