test_timezone_creation_with_arbitrary_dates fails at some times

[bmwiedemann]

Describe the bug
When running tests at certain times in the future, test_timezone_creation_with_arbitrary_dates fails.

If applicable: Stack Trace

FTBFS-log.txt

To Reproduce
Steps to reproduce the behavior:
on Debian or openSUSE do

osc checkout openSUSE:Factory/python-khal && cd $_
osc build --vm-type=kvm --noservice --clean --build-opt=--vm-custom-opt="-rtc base=2037-09-04T00:00:00" standard

Expected behavior
Tests should continue to success reliably.

OS, version, khal version and how you installed it:

• The output of khal --version: 0.11.3
• Installation method: rpm
• python version: 3.10
• OS: openSUSE-Tumbleweed 20240329

Additional context

As part of my work on reproducible builds for openSUSE, I check that software still gives identical build results in the future.
The usual offset is +16 years, because that is how long I expect some software will be used in some places.
This showed up failing tests in our package build.
See https://reproducible-builds.org/ for why this matters.