Time zones of the world

Detail

Why time zones exist

• The Sun appears to move 15° of longitude each hour.
• The Earth turns 360° in 24 hours, so 360 ÷ 24 = 15° per hour. This is why there are 24 time zones [3].
• Each 1° of longitude equals 4 minutes of time.

• Every place has its own local time, set by the Sun overhead at noon. If every town kept its own local time, trains, telegraphs and trade would become confusing.

• How standard time came about:

• In 1869, Charles F. Dowd, a school principal in New York, proposed time zones. All places within one zone would keep the same clock time [4].
• US and Canadian railways adopted time zones in 1883 [4].
• In October 1884, the International Meridian Conference in Washington, D.C. adopted the meridian through the Royal Observatory, Greenwich (London) as the Prime Meridian (0°) [2][3].
• GMT (Greenwich Mean Time: the local time at 0° longitude) was adopted as the reference time for clocks around the world [4].

• The Royal Observatory had been sending Greenwich time by telegraph since 1852 [3].

• UTC (Coordinated Universal Time) is today's world reference time. It replaced GMT for scientific use. Weather maps, radar and aviation call it Zulu time (Z) [3].

• Zone offsets run from UTC −12 to UTC +12. The −12 and +12 zones meet at the International Date Line (roughly 180° longitude) [3].

The 24 major time zones

• Time zone means a belt of the Earth that keeps one standard time (one official clock time for the whole belt).
• The world has 24 major zones, each about 15° of longitude wide (Class 11 Fig. 3.6).
• Most zones differ from GMT/UTC by a whole number of hours. Minutes and seconds stay the same across zones [2].

• On the NCERT world map (Class 6 Fig. 1.8), signs show the hours added to (+, east of Greenwich) or subtracted from (−, west of Greenwich) GMT.

• Zone boundaries are NOT straight meridians.

• Lines on the map bend and zig-zag to follow international borders and internal borders (states, provinces).
• This keeps one country or one state on a single time.
• So the statement "time-zone lines are meridians" is FALSE (Class 6 true-false item).
• Local governments decide where the boundaries go, and in many places they move far away from the ideal 15° lines [2].

Countries with many time zones

Countries that stretch far from east to west need more than one zone.

Country	Number of zones	Note
France	12 (most in the world)	Counts overseas territories (Pacific, Indian Ocean, Caribbean) — beyond NCERT
Russia	11	About 9,000 km wide, from Kaliningrad to the Bering Strait
USA	6 (Class 6) / 7 (Class 11, India: Location)	The count changes depending on whether overseas territories such as Guam and American Samoa are included
China	1	About 60° of longitude wide, but uses one time (see below)
India	1 (IST, UTC+5:30)	About 30° of longitude wide (68°7'E–97°25'E)

Half-hour and quarter-hour time zones (beyond NCERT)

• Half-hour and quarter-hour time zones are zones whose clock time differs from UTC by part of an hour (30 or 45 minutes), not a whole hour.
• Why they exist: a country picks a central meridian that falls between two whole-hour meridians, so that clock noon stays close to solar noon (the moment the Sun is highest) across its territory.

Country / area	Offset from UTC	Type
Iran	+3:30	Half-hour
Afghanistan	+4:30	Half-hour
India, Sri Lanka	+5:30	Half-hour
Nepal	+5:45	Quarter-hour
Myanmar	+6:30	Half-hour
Chatham Islands (New Zealand)	+12:45	Quarter-hour

• India's case:
• IST is based on the 82°30'E meridian, which passes near Mirzapur (Uttar Pradesh).
• 82.5° × 4 minutes = 330 minutes = 5 hours 30 minutes ahead of GMT.

• CSIR-National Physical Laboratory (NPL), New Delhi is the legal keeper of IST. It is responsible for producing, maintaining and sending out IST [5].

• India's east–west time gap:

• India covers about 20° of longitude between Gujarat and Arunachal Pradesh, which is nearly 2 hours of solar time [6].

• Proposals have been made for two zones: an Eastern Indian Time (one hour ahead) from Arunachal Pradesh to Bihar, and a Western Indian Time (one hour behind) from Uttar Pradesh to Gujarat. India still keeps a single time [6].

• "One Nation, One Time":

• The Legal Metrology (Indian Standard Time) Rules, 2026 were notified by the Department of Consumer Affairs on 27 August 2026. They were published in the Gazette on 29 August 2026 and come into force 180 days after publication [7].
• They make IST the common, legally recognised and traceable time reference for the country [7].
• Five Regional Reference Standard Laboratories keep secondary time scales linked to NPL's IST: Ahmedabad, Bengaluru, Bhubaneswar, Faridabad, Guwahati [7].
• IST is sent out through network systems (NTP, PTP) and satellite systems, including NavIC [7].

China's single time zone — the reverse case

• China's single time zone: China spans about 60° of longitude, which is about five natural zones. It still uses only one time, Beijing Time (UTC+8).
• Cause → effect:
• Beijing Time is set for eastern China, around 120°E.
• In western Xinjiang (about 75–90°E), clock time runs far ahead of the Sun.
• As a result, the Sun can rise around 10 a.m. in winter.

• Many locals unofficially keep Xinjiang Time (UTC+6), two hours behind Beijing.

• Compare with India: India has a similar problem on a smaller scale. In the North-East the Sun rises very early, so office hours waste morning daylight.

Daylight saving time (beyond NCERT)

• Daylight saving time (DST) means moving clocks one hour forward in summer, so that more daylight falls in the evening, after work.
• Where it is used: middle and high latitudes, mainly most of Europe and North America. Summer days there are much longer than winter days.

• The USA first brought in DST in 1918 [4].

• How it works:

• UTC itself never changes for DST. Only local clocks shift.

• For example, 00Z is 6:00 p.m. Central Standard Time in the USA, but 7:00 p.m. Central Daylight Time [3].

• Why India does not use it:

• India lies at low latitudes (8°4'N–37°6'N), so day length changes little through the year.

• India tried war-time clock changes in the 1940s, but it has no DST today.

• The debate:

• DST's energy-saving claim is disputed.
• The EU has debated ending seasonal clock changes (verify current status).

Prelims Hooks

• 360° ÷ 24 hours = 15° per hour, so 1° = 4 minutes, and there are 24 major time zones.
• The International Meridian Conference was held in Washington, D.C. in 1884 and adopted Greenwich as 0°. It was not held in London.
• Trap: "time-zone boundaries follow meridians exactly" is FALSE, because they bend along international and internal borders.
• Most time zones: France (12, with overseas territories), then Russia (11). USA: 6 (Class 6) or 7 (Class 11).
• Quarter-hour offsets: Nepal +5:45 and Chatham Islands +12:45. Half-hour offsets: India/Sri Lanka +5:30, Afghanistan +4:30, Iran +3:30, Myanmar +6:30.
• Order from west to east by offset: Iran (+3:30) → Afghanistan (+4:30) → India (+5:30) → Nepal (+5:45) → Myanmar (+6:30).
• China is about 60° wide but uses one time zone (UTC+8). Locals in Xinjiang unofficially keep UTC+6.
• IST meridian is 82°30'E, near Mirzapur. IST is kept by CSIR-NPL, New Delhi [5].
• Legal Metrology (IST) Rules, 2026 support "One Nation, One Time". IST is sent out via NavIC, NTP and PTP [7].
• India has no DST because day length varies little at low latitudes.

Mains Points

• Why time zones are political as well as geographic: boundaries follow borders, not meridians. Countries choose between national unity (China and India, one zone each) and matching clocks to the Sun (Russia, USA, France with many zones). Odd offsets such as Nepal's +5:45 also show national identity.
• India's single-time debate (GS-I/GS-III):
• India spans about 2 hours of solar time [6].
• In the North-East, sunrise comes early, so morning daylight is lost to office hours, and more electricity is used for evening lighting. This affects productivity and energy use.
• Arguments for two zones must be weighed against confusion in railways, aviation and banking, and the risk of dividing the nation.

• The 2026 IST Rules point to one legally traceable national time [7].

• Precise time as infrastructure (GS-III): power grids, telecom, stock markets, digital payments and cyber-forensics need synchronised time. Regional laboratories and NavIC-based distribution make IST part of India's critical infrastructure [7].

• Daylight saving time as a policy tool: it is useful at mid and high latitudes. Evidence on energy savings is mixed, and it disrupts health and schedules, which is why the EU has reconsidered it. It is not relevant to India's low latitudes.