Indian Standard Time

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, in Shankargarh Fort, Mirzapur (25°09′N 82°35′E / 25.15°N 82.58°E / 25.15; 82.58) (in Allahabad district in the state of Uttar Pradesh) which is nearly on the corresponding longitude reference line.

In the tz database, it is represented by Asia/Kolkata.

History

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 1948 and 1955, respectively. The Central observatory was moved from Chennai to a location at Shankargarh Fort Allahabad district, so that it would be as close to UTC +5:30 as possible.

Standard time

Standard time is the synchronization of clocks within a geographical area or region to a single time standard, rather than using solar time or a locally chosen meridian (longitude) to establish a local mean time standard. Historically, the concept was established during the 19th century to aid weather forecasting and train travel. Applied globally in the 20th century, the geographical areas became extended around evenly spaced meridians into time zones which (usually) centered on them. The standard time set in each time zone has come to be defined in terms of offsets from Universal Time. In regions where daylight saving time is used, that time is defined by another offset, from the standard time in its applicable time zone(s).

The adoption of standard time, because of the inseparable correspondence between time and longitude, solidified the concepts of halving the globe into an eastern and western hemisphere, with one prime meridian (as well its opposite International Date Line) replacing the various prime meridians that had previously been used.