The timing for a MedicationDispense that did not occur for a reason should follow anticipated change to the R5 version of Medication Dispense (https://jira.hl7.org/browse/FHIR-32803). This statusChange (dateTime) extension should be used for the timing of the MedicationDispenseNotDone profile.
And, change the mapping table for QI Core "Medication, Dispensed" negation rationale for relevant dateTime to the new extension:statusChange.

Also to enable cumulative medication duration referencing MedicationDispense timing when whenPrepared and whenHandedOver is not populated, a specific dateTime also needs an extension. That timing may be different in that it is the first recording of the MedicationDispense and therefore, the extension:recorded may work - see tracker to Pharmacy for this request (https://jira.hl7.org/browse/FHIR-32804).