Before 1884, every city kept its own local time based on the position of the sun. When it was noon in London, it was 12:01 in a town a few miles east. This was fine when travel was slow. The railroad changed everything — suddenly trains were arriving and departing on schedules that covered hundreds of miles, and every station had a different clock. The International Meridian Conference of 1884 established a global system of 24 time zones. Here is how it works.
Why 24 Time Zones?
The Earth rotates 360° every 24 hours — that is 15° per hour. So the world is divided into 24 zones, each roughly 15° of longitude wide. As you move east, you add one hour per zone. As you move west, you subtract one hour. Simple in theory.
What Is UTC?
UTC (Coordinated Universal Time) is the world's primary time standard. It replaced GMT (Greenwich Mean Time) as the reference point, though the two are nearly identical for everyday purposes. Every time zone is expressed as an offset from UTC — for example, UTC+5:30 is India, UTC-5 is Eastern Standard Time in the US.
UTC itself never changes for Daylight Saving Time. It is the stable anchor that everything else is measured against.
GMT vs UTC: For most practical purposes they are the same. The technical difference is that GMT is a time zone and UTC is a time standard maintained by atomic clocks. When someone says "GMT" they usually mean UTC+0.
Major Time Zones at a Glance
| City | Time Zone | UTC Offset (Standard) |
|---|---|---|
| London | GMT / BST | UTC+0 / UTC+1 |
| New York | EST / EDT | UTC-5 / UTC-4 |
| Los Angeles | PST / PDT | UTC-8 / UTC-7 |
| Paris / Berlin | CET / CEST | UTC+1 / UTC+2 |
| Dubai | GST | UTC+4 |
| Mumbai | IST | UTC+5:30 |
| Singapore | SGT | UTC+8 |
| Tokyo | JST | UTC+9 |
| Sydney | AEST / AEDT | UTC+10 / UTC+11 |
What Is Daylight Saving Time?
Daylight Saving Time (DST) is the practice of moving clocks forward by one hour in spring (to shift daylight to the evening) and back by one hour in autumn. Roughly 70 countries observe DST, but not all — Japan, India, China, and most of Africa do not. The US and Europe observe it, but on different dates, which creates brief periods where the offset between them changes.
This is why "New York to London is always 5 hours" is wrong. During the brief windows when one region has switched clocks and the other has not, it is temporarily 4 or 6 hours.
Why Are Some Offsets Non-Integer Hours?
India is UTC+5:30. Nepal is UTC+5:45. Australia uses UTC+9:30. These exist because countries chose offsets based on political and geographic reasons rather than pure 15° longitude math. India is wide enough to cover two standard zones, but chose a single national time that splits the difference. It creates scheduling confusion, but simplifies internal coordination.
The International Date Line
On the opposite side of the globe from the Prime Meridian (UTC+0 at Greenwich) is the International Date Line, running roughly along 180° longitude. Cross it going east and you subtract one day. Cross it going west and you add one day. Samoa moved to the west side of the line in 2011, skipping an entire day — December 30 simply did not exist in Samoa that year.
Convert time between any two cities instantly — with live world clock
Use the Time Zone Converter →Key Takeaways
- 24 time zones, each roughly 15° of longitude wide, based on Earth rotating 360° in 24 hours
- UTC is the universal reference standard — all zones are expressed as UTC+ or UTC−
- DST shifts clocks by +1 hour in summer; not all countries observe it
- Some zones use 30 or 45-minute offsets for political and geographic reasons
- The International Date Line is where the calendar day changes