The specification states

Using a definite-quantity duration above seconds in a date/time arithmetic calculation will result in a run-time error.

This seems to me like it would cause problems since many data models may have properties that are defined as quantities that only support UCUM units (or more specifically, don't explicitly support CQL units).  Enforcing the rule above could result in run-time errors for any mathematical calculations that use duration-based quantities from the data model.

For example, FHIR MedicationRequest has a dispenseRequest property that contains an expectedSupplyDuration property which is a Duration.  By definition that duration must be defined using UCUM units (and will often be expressed in units greater than seconds).  So... any CQL logic that attempts to add dispenseRequest.expectedSupplyDuration to a dispense date (for example) will fail with an error based on the language in the spec above.  This doesn't seem like an acceptable outcome.  I think this limitation needs to be removed from the spec.