It turns out that the mt:TourStartTime is also given in UTC, and
therefore cannot be used to get the timezone offset. The problem was
that my local computer's timezone was the same as the tour timezone, so
by the magic of Python's datetime.datetime.fromtimestamp (and the date
CLI util), I did not notice that the timestamp actually represents UTC.

Sadly, it currently looks like there is no way to extract the time zone
from a MyTourbook export.