Timezones Explained

Timezones are regions of the Earth that share the same standard time. The planet is divided into 24 primary zones, each roughly 15° of longitude wide, corresponding to one hour. However, political and geographic factors mean the actual boundaries are irregular, resulting in about 38 unique timezone offsets in practice.

Coordinated Universal Time (UTC) is the global time standard. All timezones are expressed as offsets from UTC (e.g., UTC+5:30 for India, UTC-5 for US Eastern). UTC replaced Greenwich Mean Time (GMT) as the international standard in 1972. While GMT and UTC are often used interchangeably, UTC is based on atomic clocks and is more precise.

The IANA (Internet Assigned Numbers Authority) timezone database — also called the tz database or Olson database — is the definitive reference for timezone rules worldwide. It uses names like "America/New_York" and "Asia/Kolkata" rather than abbreviations. This database is what your computer and phone use to display correct local times, and it's updated several times per year to reflect political changes.

Many countries adjust their clocks forward by one hour during warmer months to make better use of daylight. This practice began during World War I and remains controversial. Not all countries observe DST — notable exceptions include most of Africa, most of Asia, Arizona (US), and Queensland (Australia). DST transitions can cause confusion with scheduling, which is why using UTC for international coordination is recommended.

Not all timezone offsets are whole hours. India uses UTC+5:30, Nepal uses UTC+5:45, and the Chatham Islands (NZ) use UTC+12:45. Some timezones even use 15-minute increments. These non-standard offsets are a common source of bugs in software that assumes all offsets are whole hours.