Question 1

What is UTC and why is it used as a reference?

Accepted Answer

UTC (Coordinated Universal Time) is the world's primary time standard, maintained by atomic clocks. All time zones are defined as offsets from UTC (e.g. UTC+8, UTC-5). It does not change with Daylight Saving Time, making it the reliable reference point for international communication, computing logs, APIs, and database timestamps.

Question 2

What is the difference between UTC and GMT?

Accepted Answer

UTC and GMT (Greenwich Mean Time) are numerically the same — UTC+0 and GMT are at the same offset. However, they are different concepts. GMT is a time zone used in the UK and some countries in winter. UTC is an international time standard maintained by atomic clocks. Modern computing uses UTC; GMT is used colloquially but is technically a time zone, not a standard.

Question 3

What is Daylight Saving Time (DST) and which countries observe it?

Accepted Answer

DST advances clocks by 1 hour in spring ("spring forward") to extend daylight into evenings, then moves back 1 hour in autumn ("fall back"). Major regions observing DST: USA, Canada, most of Europe, Australia (opposite season). Countries that do NOT observe DST: Japan, China, India, most of Africa, most of South America. DST causes the UTC offset to shift by ±1 hour seasonally.

Question 4

Why is scheduling across time zones so difficult?

Accepted Answer

Several factors make cross-timezone scheduling complicated: DST starts and ends on different dates in the USA vs EU (typically 2–3 weeks apart), some countries have non-standard offsets (India is UTC+5:30, Nepal is UTC+5:45), some regions within a country may observe DST differently (parts of the USA, Australia), and the International Date Line means some city pairs can be 1 day apart.

Question 5

What is the International Date Line?

Accepted Answer

The International Date Line (IDL) is an imaginary line at approximately 180° longitude in the Pacific Ocean where the date changes. When you cross the IDL travelling west to east, you subtract a day. Travelling east to west, you add a day. This is why, for example, Sydney (UTC+10/+11) and Los Angeles (UTC-8/-7) can be separated by 18–19 hours — they are not on opposite sides of the clock, they are also on different dates.

Question 6

How does JavaScript handle time zones?

Accepted Answer

JavaScript's Date object stores time internally as UTC milliseconds since the Unix epoch (January 1, 1970 00:00:00 UTC). When displaying dates, it automatically converts to the user's local timezone using the browser's detected timezone. The Intl.DateTimeFormat API lets you format dates in any timezone. For reliable timezone handling in Node.js, use the date-fns-tz or Luxon libraries rather than the built-in Date object.

Time Zone Converter

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Daylight Saving Time (DST)

Key World Time Zones

Time Zone Converter Use Cases

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