On Mon, Feb 5, 2018 at 10:31 PM, Howard Hinnant <howard.hinnant@gmail.com> wrote:

    utc_offset == local_time - utc_time

This is an algebraic equation that _must_ be true.

That presumes that "utc_time", "local_time", and "utc_offset" are useful concepts.  Jettison them and the problem goes away.