code: function coerceDate (value) {
    if (!(value instanceof Date)) {
      // Is this how you raise a 'field error'?
      throw new Error('Field error: value is not an instance of Date')
    }
    if (isNaN(value.getTime())) {
      throw new Error('Field error: value is an invalid Date')
    }
    return value.toJSON()
  }
docs: /*
  Ported type from https://github.com/soundtrackyourbrand/graphql-custom-datetype