You can use a Rollup column to create totals. If you want to Rollup filtered data, then you can also apply a Rollup column to an existing relation or query instead of an entire table.
When trying to Rollup a value for a specific month, it’s best to use a math column to convert any dates into an easy numeric value.
This in a math column will give you the year and month joined together.
Year(Date)*10^2 +Month(Date)
Then you can use that number to create relations.