A time zone is a region of the world that observes a uniform standard time for legal, commercial, and social purposes. The Earth is divided into 24 primary time zones, each roughly 15 degrees of longitude wide, corresponding to one hour of the day. However, the actual boundaries of time zones are determined by countries and regions for practical reasons — political borders, economic ties, and geographic convenience all influence where one zone ends and another begins.

All time zones are defined relative to Coordinated Universal Time (UTC), the world's primary time standard. Zones are expressed as UTC+ or UTC− followed by the number of hours (and sometimes minutes) of offset. For example, UTC+5:30 is used by India, and UTC−5 is used by the US East Coast in winter.

Daylight Saving Time (DST) Explained

Many countries shift their clocks forward by one hour in spring and back by one hour in autumn to make better use of natural daylight during longer summer days. This practice, known as Daylight Saving Time (DST) or Summer Time, effectively moves an hour of morning daylight to the evening.

Not all countries observe DST. Major countries that do not use daylight saving include: the UAE, Saudi Arabia, Qatar, Japan, China, India, Singapore, and most of Africa. Countries that do observe DST include the United States, Canada, most of Europe, Australia, New Zealand, and parts of South America.

The dates of DST transitions differ between regions. The US switches clocks on the second Sunday of March and first Sunday of November. The EU uses the last Sunday of March and last Sunday of October. Australia's DST runs in reverse (spring is October, autumn is April). These differences mean the gap between two cities can change by 1–2 hours several times a year, depending on which DST rules apply to each.

Common Time Zone Abbreviations

Abbreviation	Full Name	UTC Offset
GMT	Greenwich Mean Time	UTC+0
UTC	Coordinated Universal Time	UTC+0
EST / EDT	Eastern Standard / Daylight Time	UTC−5 / UTC−4
CST / CDT	Central Standard / Daylight Time	UTC−6 / UTC−5
PST / PDT	Pacific Standard / Daylight Time	UTC−8 / UTC−7
CET / CEST	Central European (Summer) Time	UTC+1 / UTC+2
GST	Gulf Standard Time	UTC+4
IST	India Standard Time	UTC+5:30
SGT	Singapore Standard Time	UTC+8
JST	Japan Standard Time	UTC+9
AEST / AEDT	Australian Eastern (Daylight) Time	UTC+10 / UTC+11

The IANA Time Zone Database

The IANA Time Zone Database (also called the Olson database or tzdata) is the authoritative source for time zone rules used by virtually every operating system, programming language, and browser in the world. It contains historical and current rules for every time zone on Earth, including all past DST transitions and offset changes. MyZoneTime uses this database to ensure all clocks and conversions are accurate — including edge cases like Samoa's 2011 calendar skip, Morocco's Ramadan DST suspension, and Lord Howe Island's unusual 30-minute DST shift.