Calculate difference of 2 values from same table - different date ranges

Hi all - still a pretty new user to PBI and DAX but enjoying the learning.

I am trying to calculate the difference of 2 date ranges within the same table containing summed numerical data.

I have one date table and one fact table containing daily numerical data. I have been able to assign summed daily values to 2 separate date ranges through the use of 2 card visuals and 2 date slicers by editing their interactions. However I cannot find a way to calculate the difference between the 2. I have tried using 2 separate measures and a third "difference" measure but that is not viable.

I have also tried using 2 date tables and using the USERELATIONSHIP function to relate the 2nd date table to my second date range, still not working.

If anyone has any ideas that would be a great help.