Timezone Converter — convert time across cities with DST

A wall-clock time on its own (say 14:00 on 11 May 2026) is ambiguous without a zone — it points to a different moment in Colombo than it does in London. The converter handles this in two deterministic steps:

1. Resolve the entered date and time, plus the sourcezone, to a single UTC instant (epoch milliseconds). The browser's Intl.DateTimeFormat knows the historical offset of every IANA zone — including DST shifts — and the conversion uses that lookup directly.
2. Format the same UTC instant in the targetzone's local clock. Because both steps share one instant, the conversion is exact: the answer is what an atomic clock would read.

Daylight saving is handled correctly because offsets are looked up at the actual instant, not at the current moment. Converting 2 pm New York time in July uses EDT (UTC−4); the same wall clock in January uses EST (UTC−5). The same logic applies to Australia's southern-hemisphere DST, the UK's BST, central European CEST, and so on.

Sri Lanka itself sits on Asia/Colombo, fixed at UTC+5:30 with no daylight saving. The IANA database also knows the country's 1996–2006 stint at UTC+6, so historical conversions stay accurate.

Two edge cases get explicit handling. On spring-forward days, a wall clock can skip an hour entirely (02:30 didn't exist in New York on 8 March 2026). On fall-back days, the same wall clock occurs twice. The converter resolves both by picking the later offset — the same default disambiguation rule used by the upcoming Temporal.ZonedDateTime API.

Everything runs client-side. The browser ships the full IANA tz dataset and the ECMAScript Internationalization API surfaces it through formatToParts(). That means no network request, no server round-trip, and the calculator works the same on a flaky connection as on a fast one.

• IANA Time Zone Database (tzdata) — the canonical source for global offset rules
• ECMA-402 — Intl.DateTimeFormat specification (timeZone option)
• MDN — Intl.DateTimeFormat.prototype.formatToParts()
• TC39 Temporal — ZonedDateTime disambiguation rules (informative)

Time-zone offsets and DST transitions were last cross-checked against the IANA tzdata edition shipped in current Chrome/Firefox/Safari on 2026-05-11. The data refreshes through your browser's own update cycle.