why do we consider greenwich time while calculating time for various places in the world
As some parts of the world face the Sun and some don't at a given time, the places have different time. So, we needed a standard way to relate them so that there is no confusion among countries. The Greenwich meridian line passes through the town of Greenwich in London and based on how many degrees a given place is east or west from the meridian, the place's time is calculated. The place that will be 180 degrees faraway i.e. the opposite side of the world from a place will have 12 hours difference in time. For e.g. if it is 12 noon in India then it is 12 midnight in some parts of USA and Canada. Hope it helps. Thank you.