In regards to the calculation between two record

I need a calculation between two records in my app engine. I am currently working in dev environment - sandbox and needed to get a column which gives the difference between two fields.

as a example:
 

column 1: due date - 08/31/2025
column 2: lead time - 30

output : start date: 08/31/2025 -30 = 07/31/2025

and i want to use this a trigger daily on flows. How i can get this difference in my instance. I tried using formula but not working. my instance is currently not allowing me to script anything. can anybody let me know the logic can be used?

Thanks in advance.