Following advice from literature and forum contributions I have created a master calendar with fields
Period
Year
Month
Year-Month
Quarter
PeriodID
where 'Period' is simply YYYYMM linking it to an identical field in the main data table.
The field 'PeriodID' is autonumbering the records to create a unique identifier.
My app has a Multi Box for the selction of a customer and a month.
What I have observed is that my expression to calculate the sales for a customer in the previous month
Sum({$< PeriodID = {$(=Max(PeriodID)-1)}, Year = , Quarter = , Month = , Period = >}[Sales])
only returns a value >0 when I use the fields 'Period' or 'PeriodID' for selections in the Multi Box. When I use the field 'Year-Month' the result of the expression is 0.
I have validated that Max(PeriodID)-1 returns a correct value.
