"Event" = occurrence
"Time" = measure of a sequence of events
"Horizon" = the visual illusion of the sky meeting the earth, which by definition can never happen
"Event Time Horizon" would seem to be an occurrence that can never happen in some vague time yet to come . . . .
UNLESS - you believe the world is indeed flat and the sky and earth DO meet - in which case will be a blow to the sale of suites on the Seven Seas Round the World Cruise . . . .