fhirdatetime/_datetime.py is adapted from CPython's Lib/datetime.py and is
used under the PSF License Agreement below, per PSF License Agreement
clause 2 ("PSF's License Agreement and PSF's notice of copyright ... are
retained in Python alone or in any derivative version").

Summary of changes made to the adapted source, per clause 3 ("Licensee
hereby agrees to include in any such work a brief summary of the changes
made to Python"):

- The separate ``date`` and ``datetime`` classes were merged into a single
  internal ``_DateTime`` base class, used by ``fhirdatetime.FhirDateTime``
  to represent partial-precision FHIR ``date``/``dateTime`` values.
- ``timedelta``, ``tzinfo``, and ``timezone`` are imported and used
  directly from the standard library ``datetime`` module rather than being
  redefined.
- The C-accelerated ``_datetime`` extension-module fallback was removed;
  this is always the pure-Python implementation.

--

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