Window functions (WINDOW, OFFSET, INDEX, RANK...) let you compute over a set of rows defined relative to the current row, after partitioning and sorting. They make running totals, comparisons to the previous row and rankings easier without manually filtering dates.
Example
Précédent = CALCULATE([CA], OFFSET(-1, ALLSELECTED('Date'[Mois]), ORDERBY('Date'[Mois])))