Average over 12 months using two different measures

Hi maurcoll,

Thank you for reaching out to the Microsoft fabric community forum. 

Here are the few steps which you may follow to resolve the issue :

You may calculate the total response time and occurrences for both periods separately, then find the weighted average to ensure correct representation.
Average calculation Measure :

Average Response Last 12 Months = 
VAR StartDate = DATE(2024,5,1)
VAR EndDate = DATE(2025,4,30)

VAR NewCriteriaTime =
    CALCULATE([Total Time (New)], 
        FILTER('Date', 
            'Date'[Date] >= DATE(2024,6,1) && 
            'Date'[Date] <= EndDate))

VAR NewCriteriaCount =
    CALCULATE(COUNTROWS('Date'), 
        FILTER('Date', 
            'Date'[Date] >= DATE(2024,6,1) && 
            'Date'[Date] <= EndDate))


VAR OldCriteriaTime =
    CALCULATE([Total Time (Old)], 
        FILTER('Date', 
            'Date'[Date] >= StartDate && 
            'Date'[Date] < DATE(2024,6,1)))

VAR OldCriteriaCount =
    CALCULATE(COUNTROWS('Date'), 
        FILTER('Date', 
            'Date'[Date] >= StartDate && 
            'Date'[Date] < DATE(2024,6,1)))


RETURN
DIVIDE(
    NewCriteriaTime + OldCriteriaTime,
    NewCriteriaCount + OldCriteriaCount
)

If I misunderstand your needs or you still have problems on it, please feel free to let us know.  
Hope this helps!
If the response has addressed your query, please accept it as a solution and give a 'Kudos' so other members can easily find it.

Best Regards, 
Harshitha.
Community Support Team.