Two years ago I and Alberto Ferrari spent time discussing techniques to calculate new and returning customers, and we wrote the result of our study in the New and Returning Customer article in DAX Patterns web site and book.
During one of the Mastering DAX courses, a student presented a better solution and Alberto published an article describing this approach that is much faster than the ones we used before. We also worked to backport this algorithm in the "legacy" DAX, without using new features available only in Power BI Desktop, Excel 2016, and SSAS 2016 (we call it "DAX 2015" to quickly identify the DAX version). The performances are equally amazing and you can read the full story (and the DAX code) on the Computing New Customers in DAX article on SQLBI.
In the future we will also update the DAX Patterns web site, but the idea is to wait a larger work on the existing patterns updating them using the new syntax available in DAX 2015.