Hey @dominick-miller!
This is something that is not covered thoroughly enough in the IDN/ISC documentation, in my opinion. It’s actually really only referenced as a footnote in the Search documentation: when creating a custom attribute that you want to search against as a date, you need to append DateType to the end of the attribute’s system name. This will tell the system that this attribute should be of type date, rather than the default which is string.
This gets a small mention at the bottom of the Second Level Field for Identities section of the Searchable Fields documentation, and is also discussed a bit in the Compass forum.
Once you’ve renamed the identity attribute and refreshed your identities, you should be able to use the flexible date search parameters against your custom identity attribute. Let us know if it gives you any more trouble!