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IST or Indian Standard Time is calculated on the basis of 82.5° E longitude, in Shankargarh Fort (25.15°N 82.58°E) (in Allahabad district in the state of Uttar Pradesh) which is nearly on the corresponding longitude reference line. Greenwich Mean Time or GMT is the mean solar time on the 0° meridian, passing through Greenwich Observatory, located in the suburbs of London, England. It is a former global time standard and has been replaced by Co-ordinated Universal Time or UTC - an atomic timescale - which is basically synonymous with GMT. IST time is observed in all of India and Sri Lanka. For better calculation of time the world has been divided into various Time Zones. Time Zones are a geographical world globe division of 15° each, starting at Greenwich, in England, created to help people know what time is it now in another part of the world. Based on those calculations IST has been given a time offset of UTC+05:30, which means basically that IST is 5 and a half hours ahead of GMT.