How Time Zones Work
The basic principle behind dividing the Earth into 24 zones, and why there are actually more than 24 in reality.
24 reference zones
The Earth's 360 degrees are divided across 24 hours, so time zones differ by one hour for every 15 degrees of longitude. In theory, there are 24 standard time zones.
Why there are more in reality
Some countries use offsets in 30- or 45-minute increments. India is UTC+5:30 and Nepal is UTC+5:45. National borders and political factors also mean time-zone boundaries are winding rather than straight lines.
- Whole-hour offsets: most countries
- 30-minute offsets: India, Iran, parts of Australia
- 45-minute offsets: Nepal, the Chatham Islands of New Zealand
The IANA time-zone database
Software uses IANA time-zone identifiers such as ‘Asia/Seoul’ and ‘America/New_York’. This database holds the past and present offsets and DST rules for every region.
World clocks, calendars, and flight-booking systems all calculate time based on this IANA data.