{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "type": "object",
  "properties": {
    "relativeTo": {
      "oneOf": [
        {
          "type": "string"
        },
        {
          "type": "number"
        },
        {
          "type": "object",
          "properties": {
            "$date": {
              "type": "string"
            }
          },
          "additionalProperties": false,
          "required": [
            "$date"
          ]
        }
      ]
    }
  },
  "additionalProperties": false
}