Difference Between GMT and IST: Full Form, Time Difference, and Meaning

By Aarna Tiwari|Updated : October 4th, 2022

The Difference Between IST and GMT is about five and a half hours. GMT, or Greenwich Mean Time, is the mean solar time at the Royal Observatory in Greenwich, London. Similarly, IST is the Indian Standard Time observed throughout India. All countries worldwide do not live at the same time of day. Different countries fall under different time zones and calculate their time accordingly. GMT and IST are two such time zones.

The article discusses various aspects and differences between the GST and IST in detail, along with their meaning and full form.

Table of Content

IST Full Form and Meaning

The IST Full Form is Indian Standard Tim. It is used in India and Sri Lanka and was established in 1947 after India gained its independence. The IST is calculated from a clock tower in Mirzapur with directions 25.15 degrees North Latitude and 82.58 degrees East Longitude.

However, Kolkata and Mumbai adopted it only in 1948 and 1955, respectively.

GMT Full Form and Meaning

GMT stands for Greenwich Mean Time and was established in 1675. The Royal Observatory built it to assist the mariners in determining longitude at sea. It also helped them keep a standard reference time concerning other cities of England that used to keep different local times.

The Shepherd Gate Clock displays GMT Time at the Royal Observatory in Greenwich, London. When the sun is at its highest point above the Prime Meridian, it is noon at Greenwich. It is the reference point for every time zone in the world. Some GMT countries include the UK, Iceland, Greenland, and Ghana.

Difference Between IST and GMT 

The Difference between GMTvs IST is discussed below in a tabular form.

Basis of Difference

Greenwich Mean Time (GMT)

Indian Standard Time (IST)

History

GMT was established in 1675 by the Royal Observatory to help navigators at sea.

IST was established in India in 1947 after India gained independence from British Rule.

Time Calculation

GMT is considered equivalent to UTC (Universal Coordinated Time) for UK Civil purposes, but it is deemed equal to UT1 for navigation.

The Clock tower in Mirzapur is the basis of the calculation of IST. The tower coordinates are- 25.15 degrees North Latitude and 82.58 degrees East longitude.

Accuracy

GMT calculates time-based on Sun. However, because of the earth's uneven angular velocity and tilt in its axis, noon GMT may not be the exact time when the sun reaches the highest point in the sky.

The Time and Frequency Standards Laboratory at the National Physical Laboratory in New Delhi generates the official time signal for IST.

The signals are based on atomic clocks that are synchronized with UTC.

Replacement and Sufficiency

GMT was replaced by UTC as the international time standard in 1972. UTC is based on atomic measurements and hence is more accurate.

Demand for separating India into different time zones has been raised time and again. A time zone differentiated by one hour has been requested for Northeastern regions.

Countries that follow the time zone

GMT is used in the UK and other commonwealth nations such as Australia, New Zealand, and the countries of the Eastern Hemisphere.

IST is used in India and Sri Lanka.

UT1 is the modern form of mean solar time at 0 degrees longitude. UTC and UT1 differ by up to 0.9 seconds.

>> Difference Between GMT and IST [PDF]

GMT Time

GMT or Greenwich Mean Time is the time measured on Earth's 0-degree longitude which passes through Old Royal Observatory near London. India is +5:30 hours ahead of the GMT time.

The GMT is now known as the Universal Coordinated Time (UTC).

Difference Between GMT and IST Questions

Question: Every country calculates its Standard meridian as the angular distance from the Greenwich meridian. If it is12 noon at GMT, what is the time on Indian Standard Time?

  1. 5:30 P.M
  2. 5:00 P.M
  3. 5:20 P.M
  4. 5:30 A.M

Answer: Option A

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Difference Between GMT and IST FAQs

  • The Difference Between GMT and IST is that GMT was established in 1675 by the Royal Observatory to help navigators at sea. On the other hand, IST was established in India in 1947 after India gained independence from British Rule.

  • GMT vs IST is a lot different in their calculation.GMT is equivalent to UTC (Universal Coordinated Time) for UK Civil purposes. Still, it is deemed equal to UT1 for navigation. The Clock tower in Mirzapur is the basis of time calculation for IST.

  • The Difference Between GMT and IST for accuracy is that GMT calculates time-based on the sun, which may not always be exact. On the other hand, IST time signals are based on atomic clocks synchronized with UTC.

  • The Difference Between GMT and IST concerning countries is that GMT is used in the UK and other Commonwealth nations such as Australia, New Zealand, and countries of the Eastern Hemisphere. IST is used in India and Sri Lanka.

  • The time difference between GMT and IST is of +5:30 Hours. The IST is ahead of GMT. Download GMT vs IST Difference Notes here.

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