Comment by zokier

You need to account for leap seconds literally any time you are mixing durations/timedeltas with instants/datetimes, otherwise you will get wrong answer. Also needed whenever you parse ISO8601/RFC3339 style strings. For example this (correct) timestamp fails to parse:

    >>> UTCDateTime.from_rfc3339('1998-12-31T23:59:60Z')
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "/tmp/tmp.4kBsq6IYyF/venv/lib/python3.11/site-packages/whenever/__init__.py", line 797, in from_rfc3339
        return cls._from_py_unchecked(_parse_utc_rfc3339(s))
                                      ^^^^^^^^^^^^^^^^^^^^^
      File "/tmp/tmp.4kBsq6IYyF/venv/lib/python3.11/site-packages/whenever/__init__.py", line 2420, in _parse_utc_rfc3339
        return _fromisoformat(s.upper())
               ^^^^^^^^^^^^^^^^^^^^^^^^^
    ValueError: second must be in 0..59