Using a date from a formula field in a workflow automation

Hi - I have created a formula field that returns a date based on another date field in the same module. It works and the calculated date appears correctly in the formula field.

I want to create a workflow based on the date that appears in the formula field but the formula field does not appear on the workflow "Based on which Date/Time field

this rule should execute" drop down list when creating a new workflow rule.

None of the calculated date fields in this module appear on this dropdown list

What is a work around to use the formula field return value in a workflow rule?

To visualize: I have a date field called "Close Date" with the date 12/15/22. I have created a formula field called "Warning Date" that uses subdate function to return "Close Date" - 14 days = 12/1/22. I want to create a workflow rule that executes an email send on the "Warning Date". When I try to create a workflow rule and use the Based on which Date/Time field this rule should execute drop down list, the field "Warning Date" does not appear on

the drop down list.

Thanks!

Andy