utc time string convert to Unix timestamp, and then convert back to utc date time string. But the result is not matched with the original one

Question

I expect 'utc' and 'utc_str_back' should be the same time, but result is diferent. Here is the code:

#input
utc='2023-09-20T05:04:54'

#calculation
utc_time = datetime.strptime(utc, '%Y-%m-%dT%H:%M:%S')
utc_dn = utc_time.timestamp()
utc_back = datetime.fromtimestamp(utc_dn, tz=pytz.utc)
utc_str_back = utc_back.strftime('%Y-%m-%d %H:%M:%S %Z')

#output
utc_str_back ='2023-09-19 21:04:54 UTC'

Answer 1

naive datetime in Python refers to local time, not UTC. Therefore,

utc_time = datetime.strptime(utc, '%Y-%m-%dT%H:%M:%S')

is misleading; utc_time will be interpreted as local time. That goes wrong in the moment you call

utc_dn = utc_time.timestamp()

The timestamp method will convert the datetime to UTC, then calculate Unix time (which refers to UTC).

---

How to fix it

docs:

A naive object does not contain enough information to unambiguously locate itself relative to other date/time objects.

Your application requires exactly that. So use aware datetime to avoid ambiguities. EX:

from datetime import datetime, timezone

#input
utc='2023-09-20T05:04:54'

#calculation
utc_time = datetime.strptime(utc, '%Y-%m-%dT%H:%M:%S').replace(tzinfo=timezone.utc)
utc_dn = utc_time.timestamp()
utc_back = datetime.fromtimestamp(utc_dn, tz=timezone.utc)
utc_str_back = utc_back.strftime('%Y-%m-%d %H:%M:%S %Z')

#output
print(utc_str_back)
# 2023-09-20 05:04:54 UTC

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p.s. pytz is deprecated.