calculating sum for different rows depending on column value (date information)

Hi AmenYves  ,

I created a sample pbix file(see attachment) for you, please check whether that is what you want.

1. Create a date dimension table as below

2. Create relationship among the date dimension table, first table and second table

3. Create a measure as below:

Measure = DIVIDE(SUM('First Table with information'[ACTIVE Focus Group]),SUM('Second Table with information'[Active]))

If the above one can't help you get the desired result, please provide some sample data in your tables (exclude sensitive data) with Text format and your expected result with backend logic and special examples. It is better if you can share a simplified pbix file. You can refer the following link to upload the file to the community. Thank you.

How to upload PBI in Community

Best Regards