Eastern time and Pacific time. basically broadcast stations broadcast the same thing in the US and for example it may be 4 pm in Florida and 3pm in Illinois and people need to know when the show stars in their area.
Time Zones in the USA, ET stands for Eastern Time and PT stands for Pacific Time. Eastern Time is 5 hours behind Greenwich Mean Time, while Pacific Time is 8 hours behind GMT.
# Addition Information
Four time zones for the continental United States of America.
*ET (eastern time zone, on the east coast)
*CT (central time zone)
*MT (mountain time zone)
*PT (pacific time zone)
**AT (alaska time zone), HT (hawaii time zone) : Alaska and Hawaii extend farther west than the PT zone giving our country two more zones.
@Read more: http://wiki.answers.com/Q/What_does_PT_and_ET_stand_for_in_time#ixzz1UlXOEOuT