Time Zones are political things, and move. The timezone database knows
this, and correctly interprets times as they would have been at the
time.
For example, when the Time zone is Africa/Monrovia, the offset now is
0, but the offset in 1970 was -44.5 minutes, so Time.zone.at(0) is Dec
31st, 1969, 23:15:30 and not Jan 1st, 1970 00:00:00.
Prevent this spec from randomly failing based on Zonebie's selected
timezone by comparing the offset _then_ against UTC, and predicting
whether the 60's have ended yet.
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