The History of Time Zones
Before the 19th century, every town kept its own local solar time — noon was simply when the sun was highest in the sky. This worked fine when travel was slow, but the railroads changed everything. A train schedule needed consistent times, and the patchwork of local times made this nearly impossible.
In 1884, the International Meridian Conference in Washington D.C. established Greenwich, England as the Prime Meridian — the reference point for global timekeeping. From this, the concept of standard time zones emerged: the world would be divided into 24 zones, each approximately 15° of longitude wide (since the Earth rotates 360° in 24 hours, or 15° per hour).
How the IANA Timezone Database Works
Today, time zones are managed by the IANA Time Zone Database (also called the tz database or zoneinfo). This open-source database is used by every major operating system, programming language, and application to determine the correct local time anywhere in the world.
The database uses a naming convention like America/New_York or Europe/London. This format (Continent/City) was chosen because political boundaries change, but geography stays the same. When a country changes its timezone rules, the database is updated.
Coordinated Universal Time (UTC)
UTC is the modern successor to GMT (Greenwich Mean Time). Unlike GMT, UTC is defined by atomic clocks rather than the Earth's rotation, making it far more precise. UTC is the basis for all civil timekeeping worldwide.
Time zones are expressed as offsets from UTC: UTC+5:30 means 5 hours and 30 minutes ahead of UTC (this is India Standard Time). Negative offsets like UTC-8 are behind UTC (this is Pacific Standard Time in winter).
Daylight Saving Time
Daylight Saving Time (DST) is the practice of advancing clocks by one hour during summer months to extend evening daylight. When DST is in effect, a timezone's UTC offset changes by +1 hour.
Not all countries observe DST. China, Japan, India, and most of Africa don't use DST at all. The United States, Canada, most of Europe, and parts of South America and Australia do. The dates when DST starts and ends vary by country.
UTC Offsets and Half-Hour Zones
While most time zones are whole hours from UTC, several places use 30-minute or 45-minute offsets:
- India (UTC+5:30) — uses a 30-minute offset
- Iran (UTC+3:30) — uses a 30-minute offset
- Nepal (UTC+5:45) — uses a unique 45-minute offset
- Australia/Adelaide (UTC+9:30) — uses a 30-minute offset