Oh duh I had posted earlier (but focused on the pytz package): UnknownTimezoneError / pytz quirks so in fact my patch code is incorrect (should replace GMT- with Etc/GMT+ and GMT+ with Etc/GMT-)
I am curious why we don't have the latest pytz (pytz-2017.2-py2.7.egg) in Plone 5.1b4 (which pins pytz-2016.10-py2.7.egg)