Hi, Tim,
Adding to Tim Smith's reply, the reason that other major time zones (Mountain and Pacific) aren't mentioned is that usually network programs are delayed before being sent to stations in those zones, so the programs arrive in those zones at a local time that is the same as the Eastern time first mentioned in the citation
But they don't delay them for stations in the Central time zone, since the one-hour offset there is not such a big problem with respect to familiy schedules. This custom arose years ago when the cost of the delay was substantial owing to the cost of the early video tape recorders - only delay when there was a serious need.
The result is that people in the Central time zone just got used to everything on television happening an hour earlier than elsewhere (including the late news coming on at 10:00 o'clock - rather than 11:00 o'clock as it was elsewhere - in order to fit in with the network programming). Once that social custom was estabished, the networks just stayed with it!
So a program that is noted as "8/7c" would run at these local times in the four major time zones:
Eastern 8:00 (origination time)
Central 7:00 (no delay)
Mountain 8:00 (2 hour delay)
Pacific 8:00 (3 hour delay)
Thus the time in the Central zone is the only one that needs noting in the citation.