A country may have many longitudes in between the country and their is a time gap of 4 minutes between each longitude. For example the time gap between the western point of Gujarat and the eastern most point of Arunachal Pradesh has a time gap of 2 hours. So, their is a lot of confusion in the whole country. Therefore, a standard meridian should be there in each country as it is the central longitude of the whole country so that there is equal time in all the country. 
15 4 15