Offers - Doing calculations with date attributes

Hi all, 

I'm researching the possibility of doing calculations with date attributes in AJO (PQL), such as creating a dynamic variable (parameter) for days left until a specific day (use case ex. = expiration date). I've found the documentation for "date and time functions" and "arithmetic functions", but am I able to apply these arithmetic functions directly to date attributes or is there some preliminary conversion that needs to be made to the dates? 

Specific use case: date difference ((date_1 + XX days)      ,      today)

Thanks in advance for any suggestions 😁