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It turns out there's one aspect of the accepted proposal that I
\*think\* I understand, but want to confirm: the datetime -> POSIX
timestamp -> datetime roundtrip for missing times.

If I'm reading the PEP correctly, the defined invariant for local
times that actually exist is:

dt == datetime.fromtimestamp(dt.timestamp())

No confusion there for the unambiguous times, or for times in a fold.
In the latter case, the timestamps produced match the points where the
UTC times match the local times in the "In the Fold" UTC/local
diagram.

The subtle part is the handling of the "timestamp()" method for the
"missing" times where the given time doesn't actually correspond to a
valid time in the applicable timezone (local time for a naive datetime
object).

Based on the UTC/local diagram from the "Mind the Gap" section, am I
correct in thinking that the modified invariant that also covers times
in a gap is:

dt == datetime.fromtimestamp(dt.astimezone(utc).astimezone(dt.tzinfo).timestamp())

That is, for local times that exist, the invariant "dt ==
dt.astimezone(utc).astimezone(dt.tzinfo)" holds, but for times that
don't exist, "dt.astimezone(utc).astimezone(dt.tzinfo)" will normalise
them to be a time that actually exists in the original time zone, and
that normalisation also effectively happens when calling
"dt.timestamp()".

Regards,
Nick.

P.S. Thanks to whoever drew the diagrams for the In the Fold/Mind the
Gap sections - I found them incredibly helpful in understanding the
change!