Testing location data with edge cases, not just happy paths
An address in a well mapped city center tells you almost nothing about how your system handles a rural route, a disputed border, or a query near the poles. Test the hard cases deliberately.
Historical time zone changes and why they still matter
Software that computes time zone offsets often implicitly assumes the current rule for a zone has always applied, and that assumption works fine right up until you need to correctly interpret a timestamp from before the most recent rule change, at which point it quietly produces a wrong answer that looks entirely plausible. This is a genuinely common source of subtle data corruption in systems that store or process historical timestamps across a meaningful span of time.
The reason this matters is that time zone rules, daylight saving start and end dates, standard offsets, even which zone a location belongs to, are not fixed forever. They change when governments change them, as covered elsewhere, and a timestamp recorded years ago needs the rule that was actually in effect on that specific date, not the rule in effect today, to compute the correct corresponding moment in another zone or in UTC.
This is precisely why the IANA time zone database stores a full history of rule changes for each named zone rather than only the current rule. Software built correctly on top of it can answer "what was the UTC offset for this zone on this specific historical date," which is a genuinely different and more complex question than "what is the UTC offset for this zone right now," and getting them confused is exactly how historical timestamp data quietly drifts wrong.
This shows up in concrete, practical ways. A system converting a historical log timestamp from local time to UTC for analysis needs the historical rule for that log's original date and zone, not the current one, or the conversion introduces an error that can be up to a full hour in either direction depending on the specific transition involved. A system computing someone's age or a contract's duration across dates spanning a historical rule change needs the same care, though the impact there is usually smaller. Even displaying an old timestamp back to a user in their current local time requires converting through the correct historical rule for the original zone and date, not the current one.
The practical guidance is to always compute time zone conversions using a system that is aware of historical rule changes for the specific zone and date in question, rather than one that only knows the current rule, and to be particularly careful with any process that batch-converts historical data across a time span that might include a rule change. Our time zone lookup can be queried for a specific moment in time, applying the rules that were actually in effect for that date, which is exactly the behavior needed to handle historical data correctly rather than defaulting to today's rule for every calculation regardless of which date is actually being processed.