Alberto Ferrari shows how to calculate a rolling 12-month average in DAX:
The measure we want to compute is Rolling Avg 12M, which computes the rolling average of the Sales Amount measure over the last 12 months. When you project the rolling average on a chart, the resulting line is much smoother; it removes the spikes and drops that would make it difficult to recognize a trend in sales.
Click through to see two ways to do this: via a DAX measure and then as a calculation group.