REST API POST Task PlannedStart not accepted

I want to change the description of a change task by REST API. Posting just the description structure that I want to change leads to PlannedStart and PlannedEnd missing. Giving also PlannedStart and PlannedEnd leads to "does not correspond to the required date format".

What do I do wrong?

curl -v -X PUT -H "Authorization: Basic MySecretAuthorizationString" -H "Content-Type: application/json" -s myBaseUrl.de/.../CH017749-001 --data-binary @data.txt

data.txt:

{
  "IVV_CD_ChangeAufgabe": {
    "description.structure": {"Description": [
      "Jenkins-Deploy",
      "Release for production",
      null,
      "Release: 24.0001"
    ]},
    "header": {
      "PlannedEnd": "2024-05-11T20:00:00+02:00",
      "PlannedStart": "2024-05-10T20:00:00+02:00"
    }
  }
}