Complex sentence: NBC wouldn't return the 12:30 slot, or for that matter the 12:00 hour, to the local affiliates until 1991; in 1993, NBC would briefly return to airing network programming for a period of five months in the 12:00 hour.
Split into simpler sentences: NBC wouldn't return the 12:30 slot, or for that matter the 12:00 hour, to the local affiliates until 1991. In 1993, NBC briefly returned to airing network programming for a period of five months in the 12:00 hour.