Greenwich Mountain Time, or GMT, is based out of Greenwich, England. It is viewed as the median, or starting point, for all time zones. Central Standard Time, or CST, is the time zone that runs vertically through the central United States. To convert Central Time to GMT, use basic conversion charts.
CST converts to Central Daylight Time, or CDT, when the United States is on Daylight Savings Time from late March to early November. This impacts the conversion to GMT.
Central Time is behind GMT. When the U.S. is on Standard Time, CST is six hours behind GMT. During Daylight Savings, CDT is five hours behind GMT. Therefore, to convert Central Time to GMT, add six hours to CST and five hours to CDT.
If it is 9 a.m. CST, add 6 hours to get 3 p.m. GMT. Inversely, if it is 9 p.m. CST, add 6 hours to get 3 a.m. GMT. For Daylight Savings, if it is 6 a.m. CDT, add 5 hours to get 11 a.m. GMT. Inversely, if it is 6 p.m. CDT, add 5 hours to get 11 p.m. GMT.