"GMT was established to aid worldwide oceanic navigation and is based on the rotation of the Earth. It was not until the British railway system started using GMT in the mid 1800s that GMT was adopted "for land use". And it was not until the late 1800s that GMT was adopted universally. At this time, the 24 time zones were created along with the International Date Line.
I know, you want to know how to convert GMT into your local time zone. Well, it goes like this:
During Daylight Savings Time, GMT is 4 hours ahead of Eastern Daylight Time (EDT). So if the time is 0800 GMT, it is 4:00 a.m. in New York City and Miami and 1:00 a.m. in Los Angeles. During Standard Time, GMT is 5 hours ahead of Eastern Standard Time (EST). So, 1000 GMT corresponds to 5:00 a.m. in New York City and Miami and 2:00 a.m. in Los Angeles.
Please note that GMT may be listed as Zulu (Z) or Coordinated Universal Time (UTC). "