When discussing time zones, 9/8c denotes a time when it is 9 o'clock in the Eastern time zone and 8 o'clock in the central time zone of the United States and Canada. Those times happen simultaneously, but the clock reads different times in different longitudes.
The Eastern time zone is four hours behind Greenwich Mean Time, and the central time zone is five hours behind. Greenwich Mean Time, or GMT, is a global time standard. It is the time at the Royal Observatory in Greenwich, London, during the British winter. When it is 9 a.m. Eastern, it is 8 a.m. central, and it is 1 p.m. GMT.