its a little difficult working from a description without your twbx workbook - but directionally suggest you looking into using window_sum to total your fixed monthly transactions
window_sum(sum(fixed monthly),-12,0) will give you the last 12 months - can be used on a dynamic basis
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Another approach will be to use parameters if you want to give users option to choose the dates. I have provided a solution using the Sample Superstore dataset. Replace the fields with the appropriate field names in your dataset. See attached workbook.
Step 1: Create parameter [Select Month]
Step 2: Create parameter [Select Year]
Parameters are not dynamic. But this combination will server its purpose until 2050.
Step 3: Create calculated field [Selected Date]
Step 4: Create calculated field [Rolling 12 Months Transactions]
If you need a filter for other purposes then go the next step.
Step 5: Create calculated field [Rolling 12 Months Filter]
Hope this helps.