Time zones give specific areas on the Earth a time of day that is earlier or later than the neighboring time zones. This is because when it is day-time on one side of the Earth, it is night-time on the other side. There are 39 time zones dividing the Earth into different times, each with its own name, like the North American Eastern Time Zone. The North American Eastern Time Zone contains large cities in North America like New York City and Miami.
Greenwich Mean Time (GMT) is now called UTC (Coordinated Universal Time). All other parts of the world are offset (plus or minus) according to their longitude. Most of the zones are offset by a full hour, but there are some offset by half an hour or 45 minutes.
GMT was started in 1675 when the Royal Observatory at Greenwich was built to help ships to find their longitude at sea. It was a standard reference at a time when each city in England kept a different local time. GMT By about 1900, almost all time on Earth was in the form of standard time zones. Now, all time on Earth is in the form of time zones referred to some "standard offset" from GMT/UTC.