World Clock

The Kordu World Clock displays the live current time in multiple cities at once, ticking every second. Each card shows the time in HH:MM:SS, the local date, the IANA timezone identifier, and the current UTC offset — so you can tell at a glance not just what time it is in Tokyo, but exactly how far ahead or behind that is from UTC right now, including daylight saving shifts. Eight cities are loaded by default: UTC, New York, London, Dubai, Singapore, Tokyo, Sydney, and Los Angeles. Add any IANA timezone by name, remove ones you don't need, and your city list is remembered in your browser for next time.

When you need a world clock

Any work that crosses time zones eventually runs into the same problem: "what time is it for them right now?" A world clock answers that instantly, without mental math or DST edge cases. Common uses include:

• Scheduling meetings across distributed engineering, design, and product teams
• Hedge funds and prop desks tracking the open and close of Tokyo, London, New York, and Sydney sessions
• Customer support shift handoff between APAC, EMEA, and Americas
• Flight planning and jet-lag prep — checking arrival-time-of-day in the destination city
• Streaming and launch premieres — knowing when "3 pm PT Tuesday" lands for a viewer in Berlin or Seoul
• Family video calls with relatives abroad, especially across DST boundaries
• Esports, live-ops, and game releases that ship at a specific wall-clock time per region
• Journalists and trading floors reacting to news from markets in other time zones
• Remote workers deciding whether it is too late to message a colleague

How the tool works

Times are computed entirely in your browser using the native Intl.DateTimeFormat API. Once per second, a setInterval fires, re-reads the current Date, and re-formats every visible city card against its IANA timezone. There is no server call, no external time API, and no dependency on the network once the page has loaded. Accuracy is bounded by your operating system's clock, which on virtually every modern machine is kept within a few milliseconds of true time by NTP.

The IANA time zone database

The IANA (Internet Assigned Numbers Authority) time zone database — also known as tz, zoneinfo, or the "Olson database" — is the canonical source of truth for time zone rules worldwide. It is maintained by the time-zone community under ICANN/IANA stewardship and shipped inside every major operating system (Linux, macOS, Windows, iOS, Android), every modern programming language runtime, and every modern browser. Zone IDs follow a Region/City or Continent/City pattern such as America/New_York, Europe/London, Asia/Kolkata, Australia/Sydney, or Pacific/Auckland. The city is chosen as a representative, not as a label — America/Los_Angeles covers the entire US Pacific zone, not just Los Angeles.

Why not "EST" or "GMT"?

Three-letter abbreviations like EST, PST, CET, IST, or GMT are ambiguous and should never be used as timezone identifiers. IST means both India Standard Time (+05:30) and Irish Standard Time (+01:00). CST means Central Standard Time in the US (-06:00), China Standard Time (+08:00), and Cuba Standard Time (-05:00). Abbreviations also lose DST information — EST technically means "always -05:00", so a system that uses it blindly will be off by an hour for half the year. The IANA database fixes this by giving every location a fully qualified ID (America/New_York) that resolves to the correct offset on every date in history and the future.

UTC, GMT, and offset notation

UTC (Coordinated Universal Time) is the modern global time reference. It is practically identical to GMT for civilian purposes, but UTC is the rigorous technical standard used by NTP, aviation, and computer systems. Offsets are written as +HH:MM or -HH:MM relative to UTC: America/New_York is -05:00 in winter and -04:00 in summer; Europe/London is +00:00 in winter and +01:00 in summer (British Summer Time). Not every zone is a whole number of hours from UTC — India is +05:30, Nepal is +05:45, Newfoundland is -03:30, Chatham Islands is +12:45, and parts of Australia are +08:45. Naive "hours from UTC" math misses these half- and quarter-hour zones; the IANA database does not.

Daylight saving traps

DST is the single biggest source of bugs in scheduling logic. Zones shift forward in spring and back in fall, but the shift dates differ between hemispheres (Australian DST ends in early April while North American DST begins in mid-March), between countries (EU coordinates a last-Sunday-in-March shift; the US shifts on the second Sunday in March), and between regions within a country (Arizona opts out of US DST entirely; Queensland opts out in Australia). On the transition day itself, one local hour either does not exist (spring forward) or happens twice (fall back). Intl.DateTimeFormat handles every case automatically by reading the IANA rules — you just give it a zone ID and a Date, and it returns the correct local time.

Business-hour overlap planning

Distributed teams live and die by the size of their overlap window. San Francisco (UTC-8/-7) and Berlin (UTC+1/+2) share only about three usable hours — roughly 09:00-11:00 Pacific, which is 18:00-20:00 Central European Time. London and Singapore share a similar three-hour window in the morning UK / afternoon Singapore. Tokyo and New York barely overlap at all during working hours, which is why global teams usually rotate meeting times so the same region is not always taking calls at 06:00 or 22:00. A live world clock makes the trade-off visible: you can see at a glance who is about to start their day, who is mid-afternoon, and who should be asleep.

City-to-zone mappings that catch people out

• London is Europe/London, not Europe/GMT — GMT is an offset, not a place
• Moscow is Europe/Moscow (+03:00, no DST since 2011)
• São Paulo is America/Sao_Paulo (Brazil dropped DST in 2019)
• Beijing, Shanghai, Hong Kong, and all of mainland China use Asia/Shanghai
• Most of India uses Asia/Kolkata, not Asia/Calcutta (the old spelling is a deprecated alias)
• Dubai and most of the UAE use Asia/Dubai (+04:00, no DST)
• Auckland is Pacific/Auckland, not Pacific/New_Zealand

Compared to timeanddate or worldtimebuddy

Web tools like timeanddate.com and worldtimebuddy are excellent for one-off conversions and DST planning, but they are heavy, ad-laden, and require a round-trip to a third-party server. The Kordu World Clock is intentionally minimal: a single page that renders eight cities immediately, lets you add any IANA zone, and runs fully offline after load. Use it when you want the answer now, not after three popups and a cookie banner.

Privacy and data processing

Everything runs in your browser. No time data is transmitted; no requests are made to any time API. Your custom list of added cities is stored in localStorage on your device — it never leaves the browser and you can clear it at any time.