You can use an LOD calculation, the one i'd suggest in this case is the FIXED lod.
I'll be using a similar example to the cohort example shown here: Top 15 Tableau LOD Expressions (Practical Examples)
First I'll find the customers first order date:
Then see whether the customer's first order date (in months) matches the month of the invoice date:
Then bring this New Customer? field into the filters shelf and set it to true.
Please see the attached workbook.
New Customer growth_v10.2.twbx 15.2 KB