Question

How many hours difference is there between the GMT and IST?

Answer 1

In order to answer this question, first we will discuss the time difference between the GMT and IST. And then we will go through the explanation and the history of both GMT and IST separately.

Complete step-by-step solution:
The hour difference between GMT and IST is $5hours{\text{ }}and\,30minutes$ .
A time zone is a geographical area of the world that adheres to a consistent time standard for commercial, social, and legal purposes. Two such time zones are GMT and IST. Greenwich Mean Time (GMT) is the mean solar time observed at midnight at the Royal Observatory in Greenwich, London.
GMT (Greenwich Mean Time) is the mean solar time at the Royal Observatory in Greenwich, London, measured from midnight. It has been calculated in several ways in the past, including being calculated from noon; as a result, it cannot be used to define a specific time unless a context is provided. The Sun rarely crosses the Greenwich meridian and reaches its greatest position in the sky at noon ( $12:00:00$ ) GMT due to Earth's unequal angular velocity in its eccentric orbit and its axial tilt. This event could happen up to 16 minutes before or after noon GMT, according to the time equation. The annual average (i.e. "mean") moment of this event is noon GMT, which explains the term "mean" in "Greenwich Mean Time."
Indian Standard time (IST) is the time zone observed throughout India, with a time offset of $UTC + 05:30$ . India does not observe daylight saving time or other seasonal adjustments. The clock tower in Mirzapur near Allahabad, Uttar Pradesh, is approximately on the corresponding longitude reference line, and Indian Standard Time is derived on the basis of ${82.5^\circ }$ east longitude. The Union government adopted IST as the official time for the entire country after Independence in 1947, however Kolkata and Mumbai kept their own local time (known as Calcutta Time and Bombay Time, respectively) until 1948 and 1955. The Central Observatory was relocated from Chennai to Shankargarh Fort in Prayagraj district, so that it may be as close as possible to $UTC + 05:30$ .

Note: For Daylight Saving Time, neither UTC nor GMT change (DST). During the DST period, however, several countries that utilise GMT migrate to separate time zones.
For example, the United Kingdom does not observe GMT all year; during the summer, it observes British Summer Time (BST), which is one hour ahead of GMT.