EDT vs GMT
World is divided into many time zones and all places have a local time and also a time in relation with GMT which is known as Greenwich Mean Time. Greenwich is a place in England from where all time zones in the world are measured. It is the Prime Meridian or Greenwich Meridian (longitude zero degrees) that is the starting point for all time zones in the world. GMT is also said to be World Time and it sets current time at all places of the world. Of late however, it has been replaced by UTC (Coordinated Universal Time that is based upon atomic clock and considered to be more precise.
Earth’s zero degree line of longitude passes through Greenwich and is called Greenwich Meridian. If you are in a country east of GMT, you are ahead of the GMT. For example local time in India is GMT+5.5 hours. On the other hand west of GMT, local time is behind GMT. If you are in NY, local time in summers is GMT-4 hours and GMT-5 hours in winters.
EDT on the other hand is called Eastern Daylight Time and refers to a time zone that is 4 hours behind GMT or UTC. EDT is applied in eastern parts of US and Canada. In some record it is also called as Eastern Daylight Savings Time which is a system of saving time in a particular period of the year by advancing the clocks by an hour. EDT is mainly used in summers in eastern part of North American Continent. EDT starts at 3AM on the second Sunday of March (2AM EST) and continues till first Sunday of November. It ends on this day at 2AM EDT, which gets converted to 1AM EST. EDT is current time in Greenwich minus four hours. It means that when it is noon at Greenwich, it is 8 AM in the EDT time zone.