Thinking of heading to South America and are wondering what time it is where your headed? Well South America is divided into 3 time zones. They are GMT-3, GMT-4, and GMT-5. You probably have a couple of questions, One, is what is GMT, the next you might wonder why I counted from a high number to a low number? well when you look at the map of South America you will see that the numbers in the west, to the right are higher than the number to the left. This is because all world time starts in Greenwich, London, England at the Royal Observatory, thus answering your first question, What is GMT? "Greenwich Mean Time" other people refer to it as UTC or "Coordinated Universal Time" which is the same.
So, if as I like to say "Greenwich England is where time begins" you have to subtract 1 hour for each time zone that you move west. Thus the reason South America has GMT-3 "for 3 time zones from Greenwich England" GMT-4 and GMT-5 " for 5 time zones from Greenwich England. If you were to go east from Greenwich you would be adding 1 hour for each time zone such as GMT 1, GMT 2 and so for.
Now not to muddy the waters, several of the countries in South America will switch to daylight savings time. Usually the countries south of the Equator will switch in October or November and then switch back during the months of January through March. Reason being that the seasons are opposite south of the Equator than they are north of the Equator.