docs: def is_valid_timestamp(date, unit='millis'):
      """
      Checks that a number that represents a date as milliseconds is correct.
      """
      assert isinstance(date, int), "Input is not instance of int"
  
      if unit is 'millis':
          return is_positive(date) and len(str(date)) == 13
      elif unit is 'seconds':
          return is_positive(date) and len(str(date)) == 10
      else:
          raise ValueError('Unknown unit "%s"' % unit)