Time zones seem like they should just be "hour offsets from a center point," but the real rules — daylight saving, half-hour offsets, and a database that has to track every region's historical and future changes — are more tangled than most people realize.
UTC vs. Local Time vs. GMT
UTC (Coordinated Universal Time) is the precise, unchanging reference point every time zone is defined relative to — it's maintained by atomic clocks and never observes daylight saving. GMT (Greenwich Mean Time) is often used interchangeably with UTC, but it's technically a specific time zone (the UK's standard time) that happens to align with UTC only when the UK isn't in daylight saving time. Every "local time" you see anywhere in the world is really just UTC plus or minus a defined offset for that region.
How Daylight Saving Time Actually Shifts the Clock
Daylight saving time doesn't change a region's underlying UTC offset permanently — it temporarily shifts the local clock forward (usually by one hour) for part of the year, then shifts it back. This is why a city's UTC offset can appear to change twice a year even though nothing about its geographic position changed. Not every country observes DST, and the ones that do don't all switch on the same dates, which is exactly why a naive "just add hours" approach to time zone math breaks down for anything involving a DST-observing region.
Why Some Time Zones Are Off by 30 or 45 Minutes
Most time zones are whole-hour offsets from UTC, but a handful deliberately aren't. India uses UTC+5:30 for the entire country, splitting the difference so the whole nation shares one time rather than spanning two standard zones. Parts of Australia use UTC+9:30, and Nepal famously uses UTC+5:45. These aren't errors or rounding artifacts — they're intentional political and practical choices about how a region wants its clock to align with both its neighbors and the sun.
The Database Behind Every Correct Conversion
Every operating system and browser ships with a copy of the IANA Time Zone Database (often called "tz data"), a continuously maintained record of every region's historical and current UTC offset, plus every DST rule and the exact dates it takes effect. This is what actually makes accurate time zone conversion possible — instead of a tool hardcoding "UTC+5:30 for India," it looks up the current rule for that specific region from an up-to-date, regularly patched database.
See Live Times Across Cities
See the current, live time in major cities around the world at once — customized to your own list — with the free World Clock. Need to convert one specific time between zones instead, like for scheduling a meeting? Try the Time Zone Converter.
FAQ
What's the difference between UTC and GMT? For practical purposes they're the same time, but UTC (Coordinated Universal Time) is the precise scientific standard maintained by atomic clocks, while GMT (Greenwich Mean Time) is technically a specific time zone that happens to match UTC when the UK isn't observing daylight saving time.
How does daylight saving time affect the displayed offset? It uses your browser's built-in time zone database (the same one operating systems use), which already knows every region's daylight saving rules and switch dates — so the displayed time and UTC offset automatically adjust on the correct dates without this tool needing any manual updates.
Why are some time zones offset by 30 or 45 minutes instead of a whole hour? A handful of regions, like India (UTC+5:30) and parts of Australia, deliberately chose a half-hour or 45-minute offset rather than a whole hour, usually to sit roughly in the middle between neighboring zones or align better with the sun's position at that longitude — it's a real, intentional choice, not a display quirk.
Is any of this sent to a server? No — all the time calculations happen entirely in your browser, and your customized city list is only saved locally on your own device.