Set expression with Addmonths

[TomBond77]

Hi experts

What's wrong with this formula?

Sum(
{<[Timestamp.autoCalendar.YearMonth]=
{"$(= addmonths(date(Max(Timestamp.autoCalendar.YearMonth), 'YYYY-MMM')), -1)"}>}
Sales
)

This formula is fine:

Sum(
{<
[Timestamp.autoCalendar.YearMonth]=
{"$(= date(Max(Timestamp.autoCalendar.YearMonth), 'YYYY-MMM'))"}>}
Sales)

Thanks for your help, Tom

[TomBond77 · Accepted solution]

got it: 

Sum(
{<
[Timestamp.autoCalendar.YearMonth]=
{"$(= date(addmonths(Max(Timestamp.autoCalendar.YearMonth), -1), 'YYYY-MMM'))"}>}
Sales
)