Yea, I've noticed several cases where the meeting time is "upon adjournment". This could be at any time. 12:00 am doesn't feel right but 10:00 am, 12:00 pm, 2:00 pm, 6:00 pm, etc certainly doesn't feel right. At least with 12:00 am you could identify on the front end but I still don't like that - either way, it isn't right.

Since "when" is datetime, what about, adding another field, "when_note", or something to that effect, to the event.json schema which would allow you to attach said "upon adjournment"?

"when": {
  "type": "datetime"
}

"when_note": {
  "type": "string"
}