Time zones divide the Earth into regions that observe the same standard time. In theory, there are 24 zones, each covering 15 degrees of longitude and differing by exactly one hour. In practice, time zones follow political boundaries, creating a patchwork that is more complex than any simple grid.
The basic concept: UTC offsets
Every time zone is defined as an offset from Coordinated Universal Time (UTC). UTC+0 is the baseline, centered on the prime meridian at Greenwich, England. Moving east, each zone adds an hour: UTC+1, UTC+2, and so on up to UTC+12. Moving west, each zone subtracts an hour: UTC-1, UTC-2, down to UTC-12. To convert between two zones, you calculate the difference between their offsets. For example, if it is 3:00 PM in UTC+2 and you need the time in UTC-5, the difference is 7 hours, so it is 8:00 AM.
Half-hour and quarter-hour offsets
Not all zones use whole-hour offsets. India Standard Time (IST) is UTC+5:30, covering the entire subcontinent with a single half-hour offset. Nepal uses UTC+5:45 — one of only a few quarter-hour offsets in the world. Iran uses UTC+3:30. The Chatham Islands of New Zealand use UTC+12:45. Afghanistan is UTC+4:30. Myanmar is UTC+6:30. These fractional offsets usually result from geographical compromises — placing the zone boundary at the most practical location for the population.
Political boundaries vs geography
Time zone boundaries rarely follow neat lines of longitude. They bend around national borders, state lines, and even county boundaries. China spans five geographical time zones but uses a single zone (UTC+8) nationwide, meaning sunrise in far-western Xinjiang can be after 10:00 AM. Spain is geographically aligned with UTC+0 (like Portugal and the UK) but uses UTC+1 for political and economic alignment with central Europe. Russia has 11 time zones spanning from UTC+2 to UTC+12. The US state of Indiana did not fully adopt DST until 2006, with different counties observing different rules for decades.
IANA timezone database
The definitive source for timezone rules is the IANA Time Zone Database (also called tz or zoneinfo). It contains the full history of UTC offsets and DST rules for every region in the world, using identifiers like America/New_York, Europe/London, and Asia/Tokyo. These identifiers specify not just the current offset but all historical changes. This database is maintained by volunteers and is used by virtually every operating system, programming language, and web browser. When you see a timezone selector in an app, it is almost certainly backed by IANA data.
Daylight Saving Time and offsets
Many time zones shift their offset by one hour during part of the year for Daylight Saving Time. Eastern Time in the US is UTC-5 during standard time (EST) and UTC-4 during DST (EDT). This means a single IANA zone like America/New_York actually represents two different offsets depending on the date. About 70 countries observe DST, primarily in North America and Europe. The transition dates vary by region, creating periods where the time difference between two cities temporarily changes.
Practical tips for working with time zones
Think in UTC when coordinating across zones — it eliminates ambiguity. Use IANA zone names (America/Chicago) rather than abbreviations (CST) because abbreviations can be ambiguous (CST could mean Central Standard Time or China Standard Time). Always account for DST when scheduling future events. Use timezone conversion tools that handle DST automatically. When working with dates near the International Date Line, remember that the calendar date may differ between participants.