Problem description

Currently the attribute startsAt in Event Notification Template is mandatory. When the subscription is used in asynchronous mode and not started yet (status ACTIVATION_REQUESTED, or SUBSCRIPTION_UNPROCESSABLE as suggested in #243) the starting time is not known.
Also the name of the attribute implies a future date-time which is not the case since this timestamp is set to creation time of subscription and is always in the past (compare to QoD API specification: https://github.com/camaraproject/QualityOnDemand/blob/d6a3f239622e7a840ac049dfa8b2c74a3aea87b0/code/API_definitions/quality-on-demand.yaml#L533 ).
According to this fact maybe the description of this attribute can be changed to the past form.

Expected behavior

• startsAt should be optional
• Renaming from startsAt to startedAt
• Description of this attribute should contain only past form verb:
  instead of
  Date when the event subscription will begin/began
  I suggest
  Date and time when the event subscription has started