Time zones
How do time zones work?
Time zones convert one global instant into the local civil time chosen by each jurisdiction.
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.