How Are Time Zones Determined?
Time zones are determined by geography (longitude), politics, and history.
The Theoretical System
The Earth rotates 360 degrees in 24 hours = 15 degrees per hour. Theoretically, 24 zones each 15 degrees wide.
Why Reality Is Different
- Political borders: China spans 5 theoretical zones but uses just one (UTC+8)
- Economic ties: Spain uses UTC+1 to align with France and Germany
- Historical decisions: Many boundaries set during colonial era
Who Decides?
Each country sets its own time zones through national legislation. The IANA tz database records these decisions.
Half-Hour Zones
- India: UTC+5:30
- Nepal: UTC+5:45
- Iran: UTC+3:30
- Australia (central): UTC+9:30
Quick Answer
Theoretically by longitude (15 degrees per hour), but in practice by political decisions and historical precedent.
