> knowing that you are referencing a specific hour in physical time that only ever happened once
"Falsehoods programmers believe about time"
This assumption can break with:
* Leap seconds
* Calendar and time zone changes
* (In exotic circumstances) Relativity
I'll give you leap seconds, but at least in Python historical timezone changes are recorded in the datetime library, so the timezone at any particular time in the past has the timezone offset that was active then.