addition to improving performance, this ensures that
    `"same zone" semantics`_ are used for datetimes in the same zone.


.. _`TZ variable`:
    https://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html

.. _`"same zone" semantics`:
    https://blog.ganssle.io/articles/2018/02/aware-datetime-arithmetic.html
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