- It would be 01:30am GMT at 21:30 EST (9.30pm)
9:30pm EST is United States nomenclature, meaning the time on the eastern coast in cities such as New York. What you have to do is understand how many hours you need to add to the time in order to get the proper time in GMT.
New York City as a base for the Eastern seaboard is minus four hours, meaning that New York is four hours behind the GMT time zone. Therefore, if it is 9:30pm in New York you have to add four hours to the time to arrive at the GMT. This is how we arrived at 01:30 am.
The time given in this question makes it the next day in the GMT time zone. If one was asking what the time would be for a person to the east of GMT then you would subtract from the 9:30pm to arrive at 17:30 or 5:30pm if you use the plus four as the eastern distinction. Anything east of the GMT time zone will require you to add time. For those in the western time zone of the USA it would mean adding more time to the GMT to arrive at what time it is towards the east.
For example, in the western time zones, such as Mountain Time, you would add an additional three hours because the Mountain Time zone is three hours behind the east coast. To figure out any time and track it back to GMT all one has to know is whether it is plus or minus a certain amount of hours.