Indian Standard Time (IST) is the time observed throughout India and Sri Lanka, with a time offset of UTC+05:30. India does not observe Daylight Saving Time (DST) or other seasonal adjustments. In military and aviation time IST is designated E* ("Echo-Star").
Indian Standard Time is calculated on the basis of 82.5° E longitude, from a clock tower in Mirzapur ( ) (near Allahabad in the state of Uttar Pradesh) which is nearly on the corresponding longitude reference line.
After independence in 1947, the Indian government established IST as the official time for the whole country, although Kolkata and Mumbai retained their own local time (known as Calcutta time and Bombay Time) until 1955. The Central observatory was moved from Chennai to a location near Mirzapur, so that it would be as close to UTC +5:30 as possible.
Criticism and proposals 
The country's east–west distance of more than 2,933 km (1,200 mi) covers over 28 degrees of longitude, resulting in the sun rising and setting almost two hours earlier on India's eastern border than in the Rann of Kutch in the far west. Inhabitants of the north–eastern states have to advance their clocks with the early sunrise and avoid the extra consumption of energy after daylight hours.
In the late 1980s, a team of researchers proposed separating the country into two or three time zones to conserve energy. The binary system that they suggested involved a return to British–era time zones; the recommendations were not adopted.
In 2001, the government established a four–member committee under the Ministry of Science and Technology to examine the need for multiple time zones and daylight saving. The findings of the committee, which were presented to Parliament in 2004 by the Minister for Science and Technology, Kapil Sibal, did not recommend changes to the unified system, stating that "the prime meridian was chosen with reference to a central station, and that the expanse of the Indian State was not large."
Though the government has consistently refused to split the country into multiple time zones, provisions in labour laws such as the Plantations Labour Act, 1951 do allow the Central and State governments to define and set the local time for a particular industrial area. In Assam, tea gardens follow a separate time zone known as the 'Tea Garden Time' or Bagantime that is one hour ahead of IST.
Time signals 
Official time signals are generated by the Time and Frequency Standards Laboratory at the National Physical Laboratory in New Delhi, for both commercial and official use. The signals are based on atomic clocks and are synchronised with the worldwide system of clocks that support the Coordinated Universal Time.
Features of the Time and Frequency Standards Laboratory include:
- Four caesium and rubidium atomic clocks;
- High frequency broadcast service operating at 10 MHz under call sign ATA to synchronise the user clock within a millisecond;
- Indian National Satellite System satellite–based standard time and frequency broadcast service, which offers IST correct to ±10 microsecond and frequency calibration of up to ±10−10; and
- Time and frequency calibrations made with the help of pico– and nanoseconds time interval frequency counters and phase recorders.
IST is taken as the standard time as it passes through almost the centre of India. To communicate the exact time to the people, the exact time is broadcast over the state–owned All India Radio and Doordarshan television network. Telephone companies have dedicated phone numbers connected to mirror timeservers that also relay the precise time. Another increasingly popular means of obtaining the time is through Global Positioning System (GPS) receivers.
See also 
- "Military and Civilian Time Designations". Greenwich Mean Time (GMT). Retrieved 2006-12-02.
- "Two-timing India". Hindustan Times. 2007-09-04. Retrieved 2012-09-24.
- "Odds and Ends". Indian Railways Fan Club. Retrieved 2006-11-25.
- "India Time Zones". Greenwich Mean Time (GMT). Retrieved 2006-11-25.
- Sen, Ayanjit (2001-08-21). "India investigates different time zones". BBC News. Retrieved 2006-11-25.
- S. Muthiah (2012-09-24). "A matter of time". The Hindu. The Hindu Group. Retrieved 2006-11-25.
- "Standard Time for Different Regions". Department of Science and Technology. 2004-07-22. Retrieved 2006-11-25.
- "A matter of time". National Resource Centre for Women. Retrieved 2006-11-25.
- Rahul Karmakar (2012-09-24). "Change clock to bagantime". Hindustan Times. HT Media Group. Retrieved 2008-09-22.
- "Indian Time Today (IST)". Greenwich Mean Time (GMT). Retrieved 2006-11-25.
- "Satellites for Navigation". Press Information Bureau, Government of India. Retrieved 2006-11-25.
- National Physical Laboratory
- Evaluating two timezones and Daylight Saving Time for India, by Viral Shah & Vikram Aggarwal.