"What time is it in Tokyo?" sounds like a simple question. It is not. Most "world clock" tools that answer it are answering the wrong question — they tell you a number, when what you actually need is context: is it a weekday? Is the person you want to call at the office? Is the country in DST today?
A single time isn't enough
You almost never want one time. You want a range. "When is the overlap of my working hours with theirs?" "What does the next two weeks of overlap look like?" "If I have a call Tuesday at 4pm my time, what time is that for them?"
A row of digital clocks doesn't answer any of these. You have to do the math yourself, and humans are bad at that math, especially across DST boundaries.
DST transitions break the math
The US springs forward on the second Sunday of March. The EU springs forward on the last Sunday of March. For two weeks every spring, the offset between New York and London is 4 hours instead of the usual 5. Travelers who book recurring meetings during this window end up off by an hour and don't realize it for a week.
Australia is worse. Half the country observes DST (NSW, Victoria, Tasmania, ACT, SA), the other half doesn't (Queensland, NT, WA). Brazil dropped DST entirely in 2019. Mexico mostly dropped it in 2022. The rules change.
The IANA tzdata database — the source of truth for time zone rules — is updated 4–10 times per year as countries change DST policy. A static lookup table goes stale within months.
Cities are not the same as zones
Phoenix and Tucson are in Arizona, which doesn't observe DST. Navajo Nation, also in Arizona, does. Indiana switched policies in 2006 — different counties had been on different schedules for decades. India runs on a single half-hour-offset zone (UTC+5:30) for the whole country.
Naming a "city" implicitly picks a zone. The IANA names this correctly: America/Phoenix vs America/Denver vs America/Indiana/Indianapolis. Tools that just store "Arizona" or "Indiana" cause subtle bugs.
What actually helps
Three features that matter more than just showing the time:
- Day-of-week colors per row, so you immediately see "it's Sunday in Tokyo, don't schedule"
- Drag-to-select hour ranges, so you can mark a meeting window and see how it lands across all your zones
- Shareable URLs that encode the zones + range, so you can send a colleague a link with the same view
Our world clock shows day/work/evening/night bands per row, lets you click or drag to mark a time range, and updates DST automatically. For multi-person scheduling, the timezone overlap finder takes the same approach to N people simultaneously.