World Clock

UTC, local time, and why offsets aren't fixed

Every timezone is defined as an offset from UTC (Coordinated Universal Time) — the modern successor to GMT and the reference clock the whole internet runs on. India is UTC+5:30, New York is UTC-5 in winter, Tokyo is UTC+9. The trap is assuming those offsets are constant. They are not: many regions shift by an hour for part of the year (daylight saving), and the rules change by political decree, sometimes with only weeks' notice. That's why timezones are tracked in the IANA Time Zone Database, the authoritative dataset browsers and operating systems use — and why a hard-coded offset in a spreadsheet goes wrong twice a year.

Note the half-hour and even 45-minute offsets: India (+5:30), parts of Australia (+9:30), Nepal (+5:45). Anyone who assumes the world is divided into neat whole-hour zones will mis-schedule with these regions every time.

Daylight saving: where meetings quietly drift

Daylight saving time (DST) is the number-one cause of a recurring cross-border meeting silently moving. The reason is that countries don't switch on the same day, and some don't switch at all. The EU 'springs forward' on the last Sunday of March; the US does it two weeks earlier, on the second Sunday of March. For that two-week window every year, a London-New York call that's normally 5 hours apart is only 4 hours apart — so a meeting pinned to 'New York 9am' suddenly lands at a different London time than usual, and somebody joins an hour late.

It gets worse near the southern hemisphere, where winter and summer are flipped: when the north springs forward, parts of the south are falling back, so the offset can swing by two hours in a fortnight. The fix is to always anchor a recurring meeting to one city's local time (e.g. 'every Tuesday, 09:00 New York') and let everyone else's clock follow, rather than fixing a UTC time that drifts against people's working hours.

Scheduling across zones without the math

• Pin the other person's city, not just the offset — the city carries its DST rules, so the displayed time stays correct across the spring/autumn switches automatically.
• When you propose a time, state the zone explicitly: '3pm IST' or '15:00 Europe/Berlin', never a bare '3pm', which is the classic missed-call cause.
• Mind the date line: when it's late evening in Los Angeles it's already tomorrow afternoon in Sydney — a card showing a different date is warning you the call is on another calendar day for them.
• For a fixed countdown to a launch or deadline rather than a live wall-clock, use the Timer; to convert a specific instant to or from a Unix epoch value, use the Timestamp Converter.