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Time zone mismatch

published · September 12, 2026

A time zone mismatch is when the zone a visitor's browser reports keeps a different clock from the zone their IP address is located in, a cheap and strong signup signal.

A time zone mismatch is the disagreement between the zone a visitor's browser reports — Intl.DateTimeFormat().resolvedOptions().timeZone, which the operating system sets — and the zone the visitor's IP address is located in. The two are set by different things: the address by the network the request arrived from, the browser zone by the device the person is holding. When someone in one country routes through an address in another, the clocks disagree, and the browser is the half that is hard to fake without noticing.

What Spaw answers

Send the browser's zone as browser_timezone with a lookup (the browser endpoint takes it too) and timezone_matches answers: true when the browser's zone is the located zone or keeps the same clock right now (Paris and Berlin both do), false when the clocks differ, null when there is nothing to compare or the located zone is unknown. The located zone is timezone, settled by the curated country and region map or, for split countries such as the United States, by the nearest listed place inside the located country, which timezone_source reports.

What it does not mean

A mismatch is one signal, not a verdict: a traveller, a laptop that never had its clock changed, and a corporate gateway in another city all produce one honestly. It carries no weight in the score for that reason; it is a fact for a signup flow to weigh beside the others.

Related

markdown version: /glossary/timezone-mismatch.md

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