Sharepoint Search - Find all files older than today

I have a document library, which has a custom column "Expiry Date"

I have a process which needs to find all documents within this document library where the "Expiry Date" value is less than today's date.

I was trying to use the "Sharepoint Search" activity to achieve this, but if you choose a date field as the search criteria, you have to actually select a date, rather than being able to use a dynamic value such as a process field.

I am sure I must be missing something here, as searching based on a specific date is effectively useless - I am struggling to think of a single situation where it would be useful to filter on a fixed date.

Can anyone tell me how I can achieve this, either using the search, or some other method?

Thanks in advance for any assistance!