A table stores each row's UTC offset rather than the zone it came from. What can you no longer answer from it?

## A rule and one of its answers A zone is a dated rulebook attached to a location: for any moment it yields the displacement from UTC that was in force there. The rulebook has history in it, because the displacement changes — seasonally in many zones, by legislation in any of them, sometimes announced only weeks ahead, and occasionally corrected after the fact for dates already in the past. A displacement such as `+02:00` is one answer that rulebook gave, at one moment, for one location. Storing the answer and discarding the rule is a lossy compression, and what it loses is not the past but the ability to ask new questions. ## What survives Quite a lot, which is exactly why the shortcut is comfortable: - The moment is fully pinned. A clock reading plus the displacement in force is an unambiguous point on the world timeline, so anything about *when the event happened* is answerable. - The local clock reading at capture is recoverable, so *what did the operator's clock say* and *which local calendar day was it* are answerable for that row. - Ordering, differencing and matching on time all work, because they need only the moment. ## What does not | Question you may later want to ask | Answerable from a stored displacement? | |---|---| | What did the local clock read there at some other moment? | No — you hold one answer, not the rule | | Which location was this? | No — many locations share a displacement at any given moment | | What will the local clock read for a moment next year? | No, and the rule may change before then | | Should this row group with its region's local business day? | Only for this row's own moment | | Was this recorded under the rule in force, or by a broken producer? | No — there is nothing left to check the displacement against | The last row is the one people underestimate. A stored displacement is unfalsifiable: with the rule gone, no later process can tell a correct `+02:00` from a producer that hard-coded `+02:00` all year round. Keep the zone and that validation is available; keep only the displacement and it is not. ## The future-dated case Anything promised at a local clock reading in the future is the sharpest version of the problem. A commitment for 09:00 local a year out is not a known moment, because the rule that will map 09:00 to a moment may not have been written yet. Frozen as a moment with a displacement, it quietly becomes a promise to act at whatever local time that moment turns into after a rule change. Kept as a local reading plus its zone, it stays the promise that was actually made, and the moment is derived when it is needed. ## What to store instead 1. **The moment**, as the interior representation everything computes on. 2. **The zone of origin** as its own column, held as the location's rulebook identifier rather than as a displacement. 3. **The captured displacement, optionally**, as evidence of what the producer claimed — useful precisely because it can then be checked against the rule, which is the check you gave up by storing it alone. That triple is redundant on purpose. The moment is what you compute with, the zone is what lets you render and re-derive, and the captured displacement is evidence. Any two of them can validate the third. ## Where the shortcut comes from Two habits produce this column. The first is an upstream encoding that carries a displacement rather than a location, so the zone is lost in transit and whatever reads it stores what it received. The second is the belief that keeping everything in UTC is sufficient — true for computing and false for reporting. Once a row is normalised to UTC with no origin recorded, *revenue by local business day* and *incidents by local hour of day* become unanswerable for anything spanning more than one region, and those are exactly the questions someone asks a year later. ## What an interviewer is listening for That you can state the relationship crisply — a zone is a rule, a displacement is one of its outputs — and then say what that buys in a specific analysis rather than reciting it as a slogan. The strong answer names a question that becomes unanswerable, shows the column layout that keeps it answerable, and is honest that the shortcut costs nothing at all for the row it was captured on.