I just discovered Issue 20220 about a similar story with a tarfile test case. It looks like this is caused by the same underlying C library quirk, so I am closing this as a duplicate. Using Antoine’s reproducer there I triggered both this datetime failure and the tarfile failure:

TZ='America/New_York' ./python -m test -v test_imaplib test_tarfile test_datetime

Also Serhiy I discovered Issue 6478 about the strptime() cache which has patches trying to do a similar thing to your patch.