Expression with .ofType does not seem to support date / dateTime

I've been trying to evalue an expression:
`result = evaluate(handler.resource_file_json, "Observation.descendants().ofType(date)", [])`
On the Observation resource that contains:
`"effectiveDateTime": "1999-07-02"`
But in return I receive an empty list. It only seems to see the `effectiveDateTime` as a string.

BR,
Lukas