Triggering Workflow on all items in a document library

Hi all

I have a document library holding a list of documents with associated expiry dates. I've created a workflow that sends email to a person when the expiry date has reached. Sharepoint Designer only allows triggering of workflows when items are create, modified or start manually. How can I get sharepoint to run a workflow at on a particular schedule say at 1am each morning?

Thanks in advance

George

[437 byte] By [George] at [2008-1-2]