Filtering Gallery a Calculated Column That Can Be a Date or Text

Hello,

I have a Powerapp that has a screen where I'm trying to filter a gallery based on a calculated column. The calculated column is primarily a due date but will either display a date in mm/dd/yyyy format OR it will display some instructional text to guide the user. For example, the calculated column may display 10/10/2023 OR, based on other column values, it may say "Item is open" letting the user know that the record is still in open status.

Currently the filter is as follows, which is filtering out records where the calculated column "RetrospectiveDue" has certain text:

Filter('Process Improvement Log', RetrospectiveDue <> "Not Required", RetrospectiveDue <> "Fill in close date", RetrospectiveDue <> "Item is open")

I need some help adding to this filter a condition where RetrospectiveDue is less than or equal to today's date.

Any help is greatly appreciated!