content: {
    "$defs": {
      "Date": {
        "description": "Civil date in simplified ISO format, like 2021-05-26",
        "format": "date",
        "title": "Date",
        "type": "string"
      }
    },
    "$id": "https://gobl.org/draft-0/cal/date",
    "$ref": "#/$defs/Date",
    "$schema": "https://json-schema.org/draft/2020-12/schema"
  }