I recently noticed a discrepancy between the python-dateutil and pytz libraries when calculating offsets for dates after the last included transition in the zonefile binaries. What it boils down to is that when it runs out of transitions, python-dateutil selects the "standard" offset and pytz assumes that after the last transition the value "holds". In the southern hemisphere, this produces two different behaviors. Obviously there's no "right" answer here, since these are predictions about what time zones will be applied 20 years in the future (and even if the rules stayed the same, the "end date" is entirely artificial). Still, I wonder if it might be worth a bit of discussion and possibly a note in the Theory file? Barring a representation of the data that exposes the rules directly (in which case no such artificial limitation would be necessary), I think either fallback is defensible. Best, Paul