- The local time of different places in a country may vary according to their longitudes
- This may cause confusion and inconvenience for communication and transportation
- To avoid this problem, most countries adopt a uniform time for the whole country, which is called the standard time
- The standard time of a country is usually based on the local time of its central meridian or capital city
- For example, India has adopted the local time of its central meridian (82.5° E longitude) as its standard time, which is called the Indian Standard Time (IST)
- IST is five hours and thirty minutes ahead of GMT