code: function (value) {
      assertErr(value instanceof Date, TypeError, 'Field error: value is not an instance of Date')
      assertErr(!isNaN(value.getTime()), TypeError, 'Field error: value is an invalid Date')
      return value.toJSON()
    }
docs: Serialize date value into string
  @param  {Date} value date value
  @return {String} date as string