Date reminders with variables collection

Hello, I'm a newbies:) and I have few problems.

I have 13 lists on my site collection. In each list, I have a column of type date. I put in them the expiration date of products. I wolud like create only one workflow for all lists. The workflow will be checked each day the term of validity if the product has a date less than 30 days it sends a notification that the validity expires.

I have problem with:
- Build collection wtih all product expiration_date from all 13 lists.
- Calculate a  difference between two dates which results a number? (actual_date and expiration_date)

I will be very grateful for any help
best regards
Mateusz