docs: def date_to_datetime(d):
      """
      >>> date_to_datetime(date(2000, 1, 2))
      datetime.datetime(2000, 1, 2, 0, 0)
      >>> date_to_datetime(datetime(2000, 1, 2, 3, 4, 5))
      datetime.datetime(2000, 1, 2, 3, 4, 5)
      """
      if not isinstance(d, datetime):
          d = datetime.combine(d, datetime.min.time())
      return d