Two questions, one tool
Two different questions live under "what time is it in Tokyo." The first is simple: right now, this second, what does a clock on the wall in Tokyo read. The second comes up more often in practice: if a meeting starts at 3pm in Chicago, what time does that land in Tokyo, and is it even the same day there. World clock mode answers the first question with a live, ticking display. Convert mode answers the second: pick a date and time in one city, pick a second city, and see the exact result, including whether the date shifts forward or back a day.
Where the numbers come from
Every offset and daylight-saving transition on this page comes from your browser's IANA time zone database (the Olson database), the same source Windows, macOS, iOS, Android, and every major browser read from. That's a different approach from storing a table of UTC offsets. A stored table goes stale the moment a country changes its daylight-saving rule, which happens most years somewhere in the world. Reading the offset live from the same database the operating system uses avoids that, with nothing to update by hand.
A country name is not a time zone
"Time in Brazil" or "time in Russia" reads like one query with one answer, but both countries span multiple official time zones (Brazil has four, Russia has eleven). This page's country-level entries use whichever zone the large majority of that country's population and searches for that country actually mean: Brasília time for Brazil, Moscow time for Russia. That matches the convention most world-clock sites use, since a page trying to force one answer to cover every zone in a large country would be answering a question nobody actually asked.
Daylight saving doesn't run on the same calendar everywhere
Clocks in the Northern Hemisphere spring forward around March and fall back around November, because that's when their summer falls. South of the equator the seasons are reversed, so Sydney, Auckland, and Santiago spring forward around September or October and fall back around March or April. For part of the year that makes the gap between, say, London and Sydney smaller than it is the rest of the year, simply because the two cities are adjusting their clocks in opposite directions at opposite times. The reference table below shows this directly: Sydney sits at UTC+11 in January and UTC+10 in July, the reverse pattern of New York's UTC-5 in January and UTC-4 in July.
Not every offset is a whole hour
India Standard Time is UTC+5:30, fixed year-round with no daylight saving. The half-hour offset dates to 1906, when colonial authorities rejected an earlier proposal for two separate one-hour zones (GMT+5 and GMT+6) and picked the offset exactly between them instead. Nepal goes further, at UTC+5:45, and the Chatham Islands, a small New Zealand territory, sit at a standard offset of UTC+12:45 that shifts to UTC+13:45 during daylight saving. Newfoundland, Canada uses a half-hour offset too: UTC-3:30 standard, UTC-2:30 during daylight saving, a full 30 minutes off the rest of Atlantic Canada. Lord Howe Island, a small Australian island, goes further still. It's one of the very few daylight-saving regions anywhere that shifts its clocks by only 30 minutes instead of a full hour, moving from UTC+10:30 standard to UTC+11 during daylight saving.
Daylight-saving rules can change
Iran ended its use of daylight saving time in September 2022 and has kept a single fixed offset of UTC+3:30 year round since, after decades of switching clocks twice a year like most of its neighbors. A government sets a zone's daylight-saving rule and can change it whenever it wants, which is why this page reads the rule live instead of storing it.