Time zones
How do time zones work?
Time zones convert one global instant into the local civil time chosen by each jurisdiction.
Railways made local solar time impractical
Before standard time, towns commonly set clocks from the local Sun. That works while journeys are slow, but it becomes chaotic when railways need one dependable timetable across hundreds of miles. North American railroads adopted Standard Railway Time on 18 November 1883, and towns soon followed their shared clocks.
Offsets connect local time to UTC
A time-zone offset says how far local civil time is ahead of or behind UTC at a particular instant. UTC+9 means the local clock reads nine hours later than UTC; UTC−5 means it reads five hours earlier.
Zones are political, not neat longitude bands
The idealised world of 24 one-hour bands is only a starting point. Borders, trade, history and law produce half-hour and quarter-hour offsets, irregular boundaries and occasional permanent changes.
A zone name is better than a fixed offset
An IANA name such as America/New_York stores a history and rule set, allowing software to find the correct offset for a specific date. A bare UTC−5 cannot tell you whether a future New York date falls in standard or daylight time.
The rules are law, then data
There is no world government of civil time. Countries and regions choose their own clock rules, sometimes with little notice. The IANA Time Zone Database turns those legal and political decisions into data used by phones, browsers and servers, but a future conversion can still change if a government changes its rule.