Dax query - rolling period

Hi, 

You need to adjust your measure to take into account the last 12 months dynamically. 
First, create a measure that calculates the sales over the last 12 months.

Sales MAT =CALCULATE(SUM(MarketData[Sales Value]),DATESINPERIOD('Calendar'[Date], MAX('Calendar'[Date]), -12, MONTH))

Next, create a measure that calculates the total segment sales over the last 12 months.

Total Segment Sales MAT =CALCULATE(SUM(MarketData[Sales Value]),ALLEXCEPT(MarketData,MarketData[SEGMENT]),DATESINPERIOD('Calendar'[Date], MAX('Calendar'[Date]), -12, MONTH))

Finally, create the measure to calculate the Market Share over the last 12 months.

Market Share MAT =DIVIDE([Sales MAT],[Total Segment Sales MAT],0)

Make sure your date table has continuous months so the MAT calculation can roll over each month correctly