In military and aviation time, IST is designated E* = Echo*. Indian Standard Time is calculated on the basis of 82.5 degreeE longitude which is just west of the town of Mirzapur, near Allahabad in the state of Uttar Pradesh.

Introduction:
Indian Standard Time (IST) is the time zone used in India. It is calculated based on the 82.5°E longitude, which passes through the city of Mirzapur in Uttar Pradesh. IST is 5 hours and 30 minutes ahead of Coordinated Universal Time (UTC+5:30). Let's delve into the details of how Indian Standard Time is calculated.

Prime Meridian:
The Prime Meridian is a line of longitude that represents 0° longitude and divides the Earth into the Eastern and Western Hemispheres. The internationally recognized Prime Meridian passes through Greenwich, London, and is known as Greenwich Mean Time (GMT) or Coordinated Universal Time (UTC).

Indian Standard Meridian:
The Indian Standard Meridian (ISM) is the longitude that is used to calculate Indian Standard Time. It is set at 82.5°E, which roughly passes through the city of Mirzapur in Uttar Pradesh, India. This meridian was chosen to have a time difference of approximately 5 hours and 30 minutes ahead of GMT.

Coordinated Universal Time (UTC):
Coordinated Universal Time (UTC) is the primary time standard by which the world regulates clocks and time. It is the basis for civil timekeeping worldwide. UTC is based on atomic time and is independent of the Earth's rotation.

Calculation of Indian Standard Time (IST):
The calculation of Indian Standard Time involves adding the time difference of 5 hours and 30 minutes to Coordinated Universal Time (UTC). The process is as follows:

1. Determine the current Coordinated Universal Time (UTC).
2. Add 5 hours and 30 minutes to the UTC time.
3. The resulting time is the Indian Standard Time (IST).

Example:
Let's consider an example to illustrate the calculation of Indian Standard Time (IST):

Current Coordinated Universal Time (UTC): 10:00 AM
Add 5 hours and 30 minutes to UTC: 10:00 AM + 5 hours 30 minutes = 3:30 PM
Indian Standard Time (IST): 3:30 PM

Conclusion:
Indian Standard Time (IST) is calculated by adding 5 hours and 30 minutes to Coordinated Universal Time (UTC). The Indian Standard Meridian (ISM) at 82.5°E is used as the reference longitude for this calculation. IST is the time zone followed in India and is 5 hours and 30 minutes ahead of UTC.

Indian Standard Time is calculated on the basis of 82.5 �E longitude which is just west of the town of Mirzapur, near Allahabad in the state of Uttar Pradesh.