Properties can have several days types, as listed in https://build.fhir.org/codesystem-definitions.html#CodeSystem.concept.property.value_x_

Filters should be able to match against any of these.

So we should explain the required syntax.

I'd suggest aligning with the syntax for search (e.g., for Coding filters, use pipe separated system|code for concept filters, as we use in token-type search). Similarly for date filters like gt3020 ?