MTBF LTM Dax

Hello

I would like to do the MTBF of the last 12 months. To this end, I have made a measure with the following formulas.

mtbf =

var minutos= CALCULATE(SUM('MTTR Y MTBF'[Tiempo (min)]),DATESINPERIOD('Calendar'[Date], COALESCE(MAX('MTTR Y MTBF'[Fecha]),EOMONTH(MIN('Calendar'[Date]),-1)), -11, MONTH))

var tiempototal= CALCULATE(COUNTROWS('Calendar')*1440,DATESINPERIOD('Calendar'[Date], COALESCE(MAX('MTTR Y MTBF'[Fecha]),EOMONTH(MIN('Calendar'[Date]),-1)), -12, MONTH))

var intervenciones =(CALCULATE(COUNTROWS('MTTR Y MTBF'),DATESINPERIOD('Calendar'[Date], COALESCE(MAX('MTTR Y MTBF'[Fecha]),EOMONTH(MIN('Calendar'[Date]),-1)), -11, MONTH)))

var MTBF=IF(CALCULATE(COUNTROWS('MTTR Y MTBF'),DATESINPERIOD('Calendar'[Date], COALESCE(MAX('MTTR Y MTBF'[Fecha]),EOMONTH(MIN('Calendar'[Date]),-1)), -11, MONTH))>0, ((tiempototal-minutos)/intervenciones)/60/24)

MTBF RETuRN

The results are good for me, but the first year he does badly. This is because instead of starting to add the time from the incidence begins to count the twelve months before. I don't know how that data could be filtered, because I want to visualize it in an array. In addition, I have several sites and they all do not start on the same date.

Thank you very much in advance.