Difference Between GMT & IST - Indian Standard Time & Greenwich Mean Time

Greenwich Mean Time (GMT) and Indian Standard Time (IST) are two widely used time standards, primarily used to determine the time in various parts of the world. GMT denotes the time on the Prime Meridians (longitude 0 degrees) and is a basis for worldwide timekeeping. IST, however, is the time zone utilized by India, which is GMT plus 5 hours and 30 minutes. In other words, 5 hours and 30 minutes form the difference between GMT and IST. For this reason, when GMT shows 12:00 noon, IST would be 5:30 PM.

The topic of time zones, particularly the Difference Between GMT and IST, falls under Geography, General Studies Paper I of the UPSC Civil Services Examination. Understanding global time zones, longitude, and latitude plays a critical role in answering questions related to world geography, international relations, and the global positioning system. Questions related to time zones can also appear in Current Affairs in both Prelims and Mains.

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What is the difference between IST and GMT?

Here is a table comparing Greenwich Mean Time (GMT) and Indian Standard Time (IST):

Difference Between Greenwich Mean Time & Indian Standard Time
Aspect	Greenwich Mean Time (GMT)	Indian Standard Time (IST)
Definition	GMT is the mean solar time at the Prime Meridian (0° longitude) in Greenwich, London. It serves as the world's time reference.	IST is the time observed in India and Sri Lanka, which is 5 hours and 30 minutes ahead of GMT.
Time Zone	GMT is the baseline time zone from which all other time zones are derived. It is considered the standard for coordinating worldwide timekeeping.	IST is 5 hours and 30 minutes ahead of GMT, representing the official time in India.
Geographical Location	GMT corresponds to the time at the Prime Meridian in Greenwich, United Kingdom.	IST is followed in India and Sri Lanka, with a time offset of GMT +5:30 hours.
Time Offset from GMT	GMT is the baseline, meaning its offset from itself is 0 hours.	IST has a time offset of GMT +5:30 hours, meaning it is always 5 hours and 30 minutes ahead of GMT.
Usage	GMT is used globally as the reference time standard for all international time zones and activities like global navigation, aviation, and telecommunications.	IST is used exclusively in India and Sri Lanka and determines the daily working hours, television schedules, and other time-sensitive activities in these regions.
Daylight Saving Time (DST)	GMT does not observe Daylight Saving Time (DST). The time remains constant throughout the year.	IST does not observe DST. The time remains the same throughout the year, with no changes during any seasons.
Role in Global Navigation	GMT is used as the primary reference point for calculating time zones and for activities like aviation, shipping, and global navigation.	IST is only used within India and Sri Lanka and is not commonly referenced in global navigation or international time coordination.
Standardization	GMT serves as a universal standard for timekeeping worldwide, and all other time zones are calculated based on GMT.	IST is specific to India and Sri Lanka and is not used globally for standardization.
Seasonal Changes	Since GMT does not follow Daylight Saving Time, it stays fixed all year round.	IST does not change with seasons either, so there is no adjustment for daylight saving.
Impact on International Communication	GMT is crucial for international communication and coordination, especially in telecommunications, where precise timing is required.	While IST is vital for local communication and business within India, it does not have a global impact like GMT.

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What is Greenwich Meantime?

Greenwich Mean Time (GMT) is the time zone based on the mean solar time at the Royal Observatory in Greenwich, London, UK. It is the standard time used in the UK during the winter months and the reference time for many other time zones worldwide. GMT is defined as the time at the Prime Meridian, which is the imaginary line that runs from the North Pole to the South Pole and passes through the Royal Observatory in Greenwich. The Prime Meridian is used as the basis for measuring longitude and is defined as 0 degrees longitude.

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What is Indian Standard Time?

Indian Standard Time (IST) is the time zone used in India, which is 5 hours and 30 minutes ahead of Coordinated Universal Time (UTC+5:30). IST is used throughout India, including major cities such as New Delhi, Mumbai, Kolkata, and Chennai. IST was established in 1947, following India's independence from British colonial rule. It replaced the previous time zone used in India, based on the local mean time of 82.5° E longitude. IST is used as the standard time in India for all official purposes, including railway and airline schedules, and is observed throughout the country. It is also the basis for the time used in neighboring countries such as Sri Lanka and Bangladesh.

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Facts About the Indian Standard Time

The Council of Scientific & Industrial Research – National Physical Laboratory (CSIR-NPL), which maintains the Indian Standard time, has proposed the need for two time zones in India – 

• IST I (UTC+5:30) for the entire country and
• IST II (UTC+6:30) for the North-Eastern region of the country. 

This is because the early sunrise in the easternmost parts of the country due to the longitudinal extent of the country causes the loss of many daylight hours there. However, the Government of India has not favored the proposal for two time zones due to its difficulties and complexities.

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