World Clock

It leans on the IANA timezone database that ships inside every modern browser and operating system — a meticulously maintained record of every region's UTC offset rules, DST transition dates, and historical changes. When London springs forward or Sydney falls back, the database already encodes the exact moment, so the displayed times and the 'vs you' offsets shift automatically. That's also why offsets between two cities aren't constant year-round: New York–London is usually 5 hours but briefly 4 during the weeks their DST changes don't align.

Politics and geography, not astronomy. Time zones are legal constructs countries choose: India picked a single national time at +5:30 to compromise across its width, Nepal sits at +5:45 partly to distinguish itself from India, and parts of Australia use +9:30. There's nothing mathematically special about whole hours — solar noon shifts continuously with longitude — so any offset works; whole hours are merely the convention most countries adopted. The database, and this clock, handle the odd ones natively.

A simple, honest heuristic: sun icon from 6:00 to 17:59 local, moon otherwise — the at-a-glance answer to 'is it reasonable to call them right now.' It deliberately doesn't compute true sunrise and sunset, which vary by season and latitude (Stockholm's June sun sets near 10pm). For the astronomical version — actual sunrise, sunset, and twilight times for a specific place and date — the sunrise & sunset calculator does the real solar math.

Anchor to one named city, never to an abbreviation: 'Thursday 3pm New York' is unambiguous, while '3pm CST' is genuinely ambiguous (Central Standard exists in North America, China, and Cuba) and 'your time' in writing invites disaster. Add the cities involved here and scan for the overlap where everyone's icon shows a sun — the honest finding for US-to-Asia scheduling is that someone's evening is unavoidable, and knowing that upfront beats pretending otherwise. Calendar invites then convert automatically for each attendee.