Set analysis does not work for previous month selection

[Nickolay1]

Dear Community,

Hopefully you will be able to help me with this question. I need to calculate a measure which corresponds to the previous month from the one that is currently selected in the filter (for example if September is selected the measure should be calculated for August). I am trying to use following formula: 

count({<[Data Month - Year]={'$(=addmonths([Data Month - Year],-1))'}>} [Batch Number])

Field [Data Month - Year] has date format as for example 01/09/23. If I use it without set expression just to show the date it works correctly: if 01/09/23 is selected the formula Addmonths([Data Month - Year],-1) shows 01/08/23

But somehow the formula to count batches shows 0:

count({<[Data Month - Year]={'$(=addmonths([Data Month - Year],-1))'}>} [Batch Number]) 

If I try to add = sign before $ it calculates total number of batches for September (currently selected month):

count({<[Data Month - Year]={'=$(=addmonths([Data Month - Year],-1))'}>} [Batch Number]) 

If I try to add currently selected month to be ignored the result is still 0:

count({<[Data Month - Year],[Data Month - Year]={'$(=addmonths([Data Month - Year],-1))'}>} [Batch Number])

Any help would be highly appreciated! Thanks a lot in advance! 

[Nickolay1]

Yes, then it shows September

[vincent_ardiet_]

And if you create a table with [Data Month - Year] as a dimension and count([Batch Number]) as a measure, without any active selection? Does it returns Aug?

[Nickolay1]

Dear all,

I have realized that the filter had an expression set for the date selection: 

=Dual(Year([Data Month - Year])&'-'&Month([Data Month - Year]),MonthStart([Data Month - Year]))

Once I have changed it to [Data Month - Year.autoCalendar.YearMonth] and also reflected it in the formula for the prev. month it started working. Thanks a lot to all for your advice!