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This is really not a corner case. Think about it. Every place with daylight saving time will experience a duplicate hour when switching from summer time to winter time. A timestamp given in local time can map to multiple UTC times.

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Except you can remove the ambiguity by stating either PDT or PST. You don't have that option in 1927 Shanghai.

Note that strftime does not produce unambiguous time zone names. The four-letter Australian time zones are printed with names such as EST and CST. Better use -0500, +0545 and the likes, as suggested by ISO 8601.

If you want to indicate a time zone you shouldn't just use the offsets - they're ambiguous in that several different time zones can have the same offset at any point in time.

Better (IMO) to use the zoneinfo ID, e.g. "Europe/London" - along with the offset as well, in case the local time is ambiguous in that time zone due to a daylight saving transition.

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