Dimension Help

[#1]

I want to group some dates in a dimension using the below expression:

if([New/Existing]>MaxString({<Date=,Month=,Year=,MonthYear=,PeriodNum={"$(=Max(PeriodNum))"}>}[MonthYear])-12,'Yes','No')

the dates I am using are :

I want to attach values to the yes or no grouping based on the formula above, so when I change date the formula is dynamically changing.

The expression above returns an invalid dimension, can anyone help me

[Gysbert_Wassenaar]

Try using Max instead of MaxString. MaxString works on text values. Subtracting 12 from a text string won't give a result.