Local time of a particular place is determined by the mid-day sun. All places on the same meridian experience noon at the same time. As the Earth rotates 360º in 24 hours, in 1 hour it would rotate 360º/24º or 15º. Similarly, the earth would take 4 minutes to rotate 1º. Therefore, when it is midday at a particular place, the time would differ by 4 minutes for each degree of longitude east or west of that meridian. As the Earth rotates from west to east, places to the east see the sun first and so the time is ahead, while places to the west see the sun later and so are behind the time. Hence, if the time at Greenwich or 0º meridian is noon, it would be 12.04 pm at 1º E and 11.56 am at 1º W and similarly it would be11 am at 15º W and 1 pm at 15º E. Thus, each country has many local times and larger the E-W extent, the more the differences in local times.

Different local times in a country create administrative problems. Timings of flights or trains and timings of radio and TV broadcasts pose need to be uniform in a country. So, a standard time is followed in many countries. In India 82º30’ East longitude, which passes through the middle of the country, is taken to be the Standard Time for India and all places in India have a uniform time. It is 5 hours 30 minutes ahead of GMT or Greenwich Mean Time. However, countries with a large longitudinal extent, like Russia, Canada or the USA, have more than one standard time.