UTC is Coordinated Universal Time, an atomic standard. All time zones are defined as offsets from UTC. Kept within 0.9 seconds of atomic clocks. In programming and aviation it's used as the single reference point.
DST — daylight saving
Not everywhere
US, EU — yes · RU, China, India — no
Daylight Saving Time shifts clocks forward by 1 hour for the summer. In effect in the US, Canada, EU, Australia. Russia abandoned it in 2011, China never used it. Because of DST, winter and summer offsets can differ: EDT (UTC-4) vs EST (UTC-5).
Non-round offsets
IST, NPT
India: +5:30 · Nepal: +5:45 · Iran: +3:30
Not every zone is a whole hour. India (IST) — UTC+5:30, Nepal (NPT) — UTC+5:45, Iran (IRST) — UTC+3:30, parts of Australia — UTC+9:30. The reason is historical: countries chose an offset that matched sun time in the capital.
IANA names
Database
Europe/Moscow · America/New_York
The canonical timezone database is IANA Time Zone Database (tzdata). Format is 'Region/City'. Accounts for historical changes: DST rules, moves, abolitions. Updated several times per year. Used across Unix systems, JavaScript, Python and most databases.
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Frequently asked questions
Pick the zones and the page shows the local time in each along with the gap between them, with world clocks for a dozen major cities below. Daylight saving is handled for you: the arithmetic goes through the timezone database built into your browser rather than through fixed offsets.
The page works it out, but the useful part is knowing the gap is not constant. Moscow and New York are seven or eight hours apart depending on the season, because the United States still shifts its clocks and Russia stopped in 2014. Memorising a single number will eventually mislead you.
UTC is the modern standard kept by atomic clocks; GMT is the historical one tied to astronomical observation at Greenwich. Numerically they agree to within a fraction of a second and nobody notices the difference in conversation, but technical documents and standards should say UTC.
Look for the overlap in working hours: the world clocks show several cities at once, so the shared window is visible without arithmetic. Remember the working week itself varies — in parts of the Middle East it starts on Sunday and Friday is the day off, which no offset calculation will tell you.
History and politics rather than astronomy. India runs the whole country on UTC+5:30, Nepal on UTC+5:45, parts of Australia on UTC+9:30. Such offsets were compromises between local solar time and the wish to keep a single clock inside one state, and they simply never got tidied up.
No. Everything is computed in your browser against its own timezone database, and the zones you choose are neither transmitted nor stored. One practical consequence: a browser that has not been updated for a long time may not know the newest daylight saving rule changes.