Standard time, GMT and UTC

Detail

Why a country needs standard time

• Local time is the time at a place set by the Sun. When the Sun is highest in the sky at that place, it is local noon there.
• Local time changes with longitude. The earth turns 360° in 24 hours.
• That is 15° in 1 hour, or 1° in 4 minutes.

• So every place on a different meridian has a slightly different local time.

• If every town kept its own local time, one country would have many clock times.

• Trains, post, telegraph and offices would all run on different clocks.

• Timetables would become confusing.

• Solution: standard time. This is one uniform clock time used by the whole country. It is taken as the local time of one chosen meridian.

Standard meridian

• Standard meridian means the meridian whose local time becomes the standard time of a country.
• By convention it is a multiple of 7°30′.
• 7°30′ of longitude = 30 minutes (half an hour) of time.
• So a country's offset from Greenwich comes out as a whole hour or a half hour, never an awkward figure like 5 h 17 min.

• Class 11 NCERT says "divisible by 150°" in one place. This is a misprint. Read it as 15° (whole hour) or 7°30′ (half hour). The Class 11 chapter "India: Location" gives 7°30′ correctly.

• India's standard meridian is 82°30′E. It passes near Mirzapur in Uttar Pradesh.

• 82°30′ = 11 × 7°30′, so it fits the rule.

• 82.5° × 4 minutes = 330 minutes = 5 hours 30 minutes ahead of Greenwich.

• Large countries running from east to west may use more than one standard meridian, which gives them several time zones. India uses one standard time for the whole country.

Greenwich Mean Time (GMT)

• Prime Meridian (0°) is the meridian that runs through the Royal Observatory at Greenwich, near London (United Kingdom). All longitudes are counted east or west from it.
• International Meridian Conference, October 1884, Washington, D.C.: it adopted the meridian of the transit instrument at the Royal Observatory, Greenwich, as the prime (zero) meridian [5].

• At this conference, GMT was adopted as the reference time for clocks around the world [7].

• Greenwich Mean Time (GMT) is the mean local time at 0°.

• "Mean" means average. The real solar day is a little longer in some months and shorter in others. GMT uses the average day, so every day is exactly 24 hours.

• GMT was the world's reference time from 1884. Every standard time was given as hours ahead of GMT (east of Greenwich) or behind GMT (west of Greenwich).

Coordinated Universal Time (UTC)

• Coordinated Universal Time (UTC) is the main world time standard today. It is the basis of all civil time across the world [7].
• Atomic clock means a clock that counts the very regular vibrations of atoms. In most such clocks the atom is caesium, which gives highly accurate time.

• The UTC second is defined by an atomic change in caesium. UTC comes from TAI (International Atomic Time) and is accurate to about 1 nanosecond (a billionth of a second) per day [7].

• Who keeps it: a worldwide set of atomic clocks. The International Bureau of Weights and Measures (BIPM) coordinates it.

• About 70 countries, including India, run atomic clocks at their national measurement institutes (NMIs) [2].

• Timeline: UTC began replacing GMT for civil use in the 1960s. It has had its present form since 1972.

• IST = UTC+5:30.

Feature	GMT	UTC
Based on	Earth's rotation (mean solar time at 0°)	Atomic clocks (caesium second)
Reference since	1884	1960s; present form 1972
Kept by	Royal Observatory, Greenwich (astronomical observation)	Worldwide set of atomic clocks, coordinated by the BIPM
Accuracy	Changes with the earth's uneven spin	About 1 nanosecond per day [7]
Use today	Name still used in common speech; UK winter time	World civil time standard

How India keeps IST

• CSIR-National Physical Laboratory (CSIR-NPL), New Delhi, is India's NMI and the keeper of IST [2][3].
• IST is kept traceable to UTC within ±2.9 nanoseconds [2].
• Its National Primary Timescale is a bank of atomic clocks: five caesium clocks and two active hydrogen masers [2].

• IST reaches users through network time services and satellite links [2].

• NavIC link: NPL provides IST to ISRO timing centres at Bengaluru and Lucknow. All NavIC (Navigation with Indian Constellation) satellite clocks match IST within a few nanoseconds [2].

• Regional timing centres: five IST-linked centres are being set up at the Regional Reference Standard Laboratories in Ahmedabad, Bengaluru, Bhubaneswar, Guwahati and Faridabad [2].
• "One Nation, One Time": the Department of Consumer Affairs runs this with NPL and ISRO. It aims to send IST across India with millisecond-to-microsecond accuracy [4].
• The Legal Metrology (Indian Standard Time) Rules, 2026 have been notified. They make IST the common, legally recognised and traceable time reference for India [4].
• Sectors that need precise time: telecom, banking and stock trading, power grids, air traffic control, navigation, 5G and cyber security [2][4].

Leap second

• The problem:
• Atomic clocks tick at a perfectly steady rate.
• The earth's rotation is not steady. It speeds up at some times and slows down at others. One cause is the braking effect of the Moon's gravity (tidal friction) [3].

• So atomic time (UTC) slowly drifts away from time based on the earth's rotation (UT1).

• Leap second means one extra second added to UTC to keep it within 0.9 second of the earth's rotation time [6].

• Leap seconds were introduced into UTC in 1972 [6].

• 27 leap seconds have been added since 1972, at gaps of six months to seven years [3].

• The last leap second came on 31 December 2016.

• It was added at 23:59:59 UTC, which was 05:29:59 IST on 1 January 2017 [3].

• NPL's time services (NTP, Teleclock) passed the corrected time to users automatically [3].

• Ending leap seconds: in 2022, the CGPM (General Conference on Weights and Measures) decided in Resolution 4 to stop adding leap seconds by 2035.

• The reason is that a sudden extra second can upset computers, networks and satellite systems that need continuous time.

Prelims Hooks

• Standard meridians are multiples of 7°30′ (half an hour). NCERT's "150°" is a misprint.
• India's standard meridian is 82°30′E, near Mirzapur (Uttar Pradesh). IST = GMT/UTC + 5:30.
• The International Meridian Conference (October 1884) met in Washington, D.C., not London. It fixed Greenwich as 0° [5].
• GMT is based on the earth's rotation (mean solar time). UTC is based on atomic clocks (caesium).
• The BIPM coordinates UTC. CSIR-NPL, New Delhi keeps IST using 5 caesium clocks and 2 hydrogen masers [2].
• Leap second: keeps UTC within 0.9 s of the earth's rotation. 27 have been added since 1972. The last was on 31 December 2016.
• CGPM 2022 (Resolution 4): leap seconds will stop by 2035.
• Trap: the earth's rotation is irregular and slowly slowing down. Atomic time is steady. It is the earth that "loses" time, not the clock.
• The Legal Metrology (IST) Rules, 2026 make IST the legal time reference ("One Nation, One Time") [4].

Mains Points

• Why standard time matters: many local times inside one country would confuse railways, communication and administration. One standard meridian (a multiple of 7°30′) gives one national clock. This is a clear example of how longitude shapes how a country is run (GS-I).
• Earth time to atomic time: GMT depended on the earth's uneven rotation. UTC (1972) depends on atomic clocks. Leap seconds were the bridge between the two. The CGPM's 2022 decision to end them by 2035 shows that steady digital time now matters more than staying in line with the Sun.
• Precise time as infrastructure (GS-III): banking, stock exchanges, power grids, 5G, air traffic control and NavIC all need time matched to nanoseconds [2]. CSIR-NPL's atomic timescale, the five regional timing centres and the Legal Metrology (IST) Rules, 2026 build national time security and help fight cyber crime [2][4].
• Debate on one time zone: India's single meridian at 82°30′E spans about 29° of longitude (68°7′E to 97°25′E). Sunrise in the far north-east comes almost two hours earlier than in the far west, so daylight is lost there in the evening. The choice is between one national time for unity and simple rules, and local daylight for working hours and energy saving.