How to get records for Deals with Closing Date in the past

I'm quite new to Zoho CRM development.

How do I get records for Deals with Closing Date in the past in a standalone Function?

I can see from the documentation this is the general form to get records:

<response>=zoho.crm.getRecords(<module_name>, [<page>],[<perPage>], {<optionalDataMap>}, <connections>);

for me:

<module_name) = "Deals"

the API name of the field is "Closing_Date"

but what is the format for the optionalDataMap? the example in documentation is only for an exact match, but I want to get only records with Closing_Date less than zoho.currentdate

l could fetch everything and then iterate over the list but that seems wasteful.

Thank you!