If you just count Customer Name by Month of Order Date , you are counting transactions , not unique customers. Worse, if someone bought in Jan and Feb, they appear in both months.
But LODs require you to think .
You are experiencing the tension between and Aggregate Level calculations. fixed calculation tableau
But when you filter the dashboard to "2024" only, that MIN changes. The customer who first bought in 2020 suddenly shows a first purchase date of 2024. The calculation is . If you just count Customer Name by Month
FIXED [Sub-Category] : MAX([Sales]) Wait, that gives me a number, not a flag. you are counting transactions
You try a standard calculation: MIN(Order Date) .