Time zones were created in the 19th century to solve the confusion caused by each town having its own local time. There are 24 main time zones, corresponding to 24 hours in a day, that are theoretically 15 degrees of longitude wide. However, in practice, time zone boundaries often follow political borders for convenience.
How time zones work
The Earth rotates 360° in 24 hours, which means it rotates 15° per hour. In theory, each timezone spans 15° of longitude, and the time advances by exactly one hour as you move from one zone to the next. The Prime Meridian (0° longitude, passing through Greenwich, London) is the reference point — UTC+0. Moving east from Greenwich, each zone is one hour ahead (UTC+1, UTC+2, etc.), and moving west, each zone is one hour behind (UTC-1, UTC-2, etc.).
Why time zone boundaries are irregular
In practice, time zone boundaries zigzag to follow national and regional borders. China, for example, uses a single timezone (UTC+8) despite spanning 5 geographic timezone widths — meaning the sun rises as late as 10 AM in western China. India uses UTC+5:30 (a half-hour offset) to center its civil time on the country's geographic midpoint. Some regions use UTC+5:45 (Nepal), UTC+8:45 (Eucla, Australia), or UTC+12:45 (Chatham Islands, New Zealand) — further proof that political and cultural factors override the mathematical 15° grid.
How many time zones are there?
Theoretically, there are 24 one-hour time zones. In practice, there are 38 distinct timezone offsets used worldwide (including half-hour and quarter-hour offsets), plus numerous DST variations. The IANA Time Zone Database — used by every major OS and programming language — tracks over 400 timezone identifiers (e.g., "America/New_York", "Asia/Kolkata") to handle every regional and historical variation.